The Legality of Iran’s Closure of the Strait of Hormuz

This post assesses the legality of the closure of the Strait of Hormuz from the perspective of the law of armed conflict at sea. In the context of the rights of neutral States, it discusses the passage regime in and over the Strait of Hormuz that continues to apply in the on-going international armed conflict (IAC) that was unlawfully launched by the United States and Israel against Iran.

The belligerents United States, Israel and Iran are not parties to the United Nations Convention on the Law of the Sea (UNCLOS). From this legal perspective, it is difficult to ascertain that the right of transit passage governs the passage and overflight of their warships and military aircraft in peacetime. The recent attacks against merchant vessels in the Strait of Hormuz are discussed mostly from the perspective of the law of targeting and naval blockade. 

Characteristics of the Strait and Attacks on Oil Tankers

Based on a systemic interpretation of Part III of the UNCLOS, an international strait is a natural sea passage that is used for international navigation between two larger maritime areas, and which is not more than 24 nautical miles wide as measured from coast to coast or from baseline to baseline and, in respect of which, international navigation is safeguarded under a treaty. The Strait of Hormuz meets this definition owing to a short section at the very centre of the strait where the territorial seas of the coastal States Iran and Oman overlap. At its narrowest the strait is 20 NM wide. 

The traffic separation scheme (TSS) in the Strait of Hormuz was adopted in 1973 and revised in 1979 by the International Maritime Organization (IMO). Thus, it is one of the oldest TSS globally. The TSS is important for ensuring the safe navigation of ships in this densely navigated maritime area that is used for transporting approximately one-fifth of the global oil demand. The sea lanes and TSS are compulsory for merchant ships, but according to the US Commander’s Handbook on the Law of Naval Operations (at p. 2-9) they are not mandatory for sovereign immune ships. Yet in practice it is still considered advisable for the warships to follow them.

Iran has used the TSS rules to arrest foreign tankers in the Strait of Hormuz before. In response to the UK interdiction of the Iranian tanker Grace 1 in the Strait of Gibraltar in 2019, a UK-flagged and Swedish-owned tanker Stena Impero was arrested by the Iranian forces in the Strait of Hormuz for the alleged violation of the TSS. Also in 2019, Iran was suspected of carrying out mine attacks which damaged a Panama-flagged and Japan-owned oil tanker as well as a Marshall Islands-flagged and Norway-owned oil tanker. In 2024, Iran arrested a Portuguese-flagged and Israeli-owned container ship MSC Aries in the strait.

In less than a week after the United States and Israeli attack, nine commercial ships have been reportedly attacked in and around the Strait of Hormuz. In this context, President Trump pledged that: ‘If necessary, the United States Navy will begin escorting tankers through the Strait of Hormuz, as soon as possible.’ If the United States or Israeli warships actually start to escort merchant vessels in the area, then under Rule 60(d) of the San Remo Manual the escorted ships may be considered as enemy merchant vessels and military objectives. Rule 61 of the San Remo Manual clarifies that any attack on these vessels is subject to the basic rules set out in its Rules 38-46. The Manual, which is currently subject to revision, is widely considered as reflecting customary international law on the law of armed conflict at sea.

In my view, it is more likely that the United States’ and Israeli warships will constantly patrol the Strait of Hormuz, which would deter Iran from attacking merchant vessels. In this context, the warships would not be escorting any merchant vessels in the Strait of Hormuz, and the ships would not be sailing in a convoy. By comparison, Russian warships recently started patrolling the Gulf of Finland to deter any Finnish or Estonian enforcement operations against its stateless and anchor-dragging tankers in the area. But the Russian mission is designed in a way to avoid a situation where Ukraine could target the merchant ships based on a claim that the ships are sailing in a convoy with an enemy warship.

In addition, the oil which is being shipped through the Strait of Hormuz on board a tanker which flies a neutral flag might turn the merchant vessel a military objective if the oil is destined to be partly used by the enemy’s military vehicles and for other military purposes. Pursuant to Article 52(2) of Additional Protocol I (the United States accepts its customary nature), Iran can target such tankers that transport oil to Israel or the United States for this oil to be used by their army to support their military campaign. Legally, such ‘import tankers’ are treated differently and they can be categorized as military objectives within the meaning of Rule 60(b) or (g) of the San Remo Manual. Based on this doctrine, a tanker that carries oil to Israel might be targeted by Iran in the Strait of Hormuz.

Is there a Blockade in the Persian Gulf?

Iran’s closure of this chokepoint of international navigation has been mistakenly characterized as a de facto blockade. A blockade is the effective prevention of access of vessels and aircraft of all States to and from specific coastal areas that are part of or under the control of an enemy State. In the Iran-Iraq armed conflict (tanker war) in 1980-1988, both belligerents imposed an undeclared blockade against each other. Currently, the Iranian closure of the Strait of Hormuz is not effective. At the very least, the United States and Israeli warships, including submarines, and military aircraft still appear able to navigate through and fly over the strait. In addition, reports suggest that some Chinese-owned tankers have continued to cross the Strait of Hormuz despite warnings.

But would Iran be legally entitled to impose a naval blockade in the Strait of Hormuz? Chiefly, this depends on whether the whole coast of the Persian Gulf (except for Iran) is part of or under the control of an enemy State, i.e. the United States. On the one hand, Iranian attacks have targeted each Arab State in the Persian Gulf. On the other hand, this can be explained by the presence of the United States military bases in each of these Arab countries. Hypothetically, if the United States would control the coastlines of Arab States in the Persian Gulf, then Iran could impose a blockade against the entire Arab coastline of the Persian Gulf. The key criterion of a blockade is effectiveness (Rule 95 of the San Remo Manual), which in principle implies that no ship or aircraft can be allowed to breach the blockade. By now, most Iranian warships have been sunk in the on-going IAC, which likely means that Iran is not able to impose an effective blockade in the Strait of Hormuz.

Fortunately for neutral States in the on-going IAC, there is no blockade in the Strait of Hormuz. Hence, from a legal perspective, whether neutral ships and aircraft have the right to continue navigating through and flying over the international strait is determined by the passage regime of the Strait of Hormuz. In respect of aircraft, this boils down to the question of whether the Strait of Hormuz is subject to the regime of transit passage or (non-suspendable) innocent passage, which does not afford the right of overflight. But even the applicability of the transit passage regime has some caveats, as explained below.

The Right of Transit Passage in the Strait of Hormuz

The Strait of Hormuz connects the EEZs in the Persian Gulf with the EEZs in the Arabian Sea, in effect meeting the criteria of Article 37 of UNCLOS for the applicability of the transit passage regime. Transit passage allows to exercise freedom of navigation and overflight subject to conditions stipulated under Part III of UNCLOS. 

The UNCLOS legal framework in respect of various maritime zones largely continues to apply in an armed conflict. Both in respect of neutral as well as belligerent States. States are required to ensure safe passage through an international strait to neutral ships and aircraft (Corfu Channel case, at p. 29).

This implies that, in principle, based on the underlying logic of the judgment in the Corfu Channel case, a strait State, nor any other party, is not entitled to temporarily close, for example, by means of laying minefields, an international strait for the navigation of neutral ships in an IAC. It is of course possible to close an international strait temporarily to the enemy’s ships and aircraft. For example, Egypt closed the Strait of Tiran to Israeli ships in 1956 and 1967. But even if Iran lays naval mines in the Strait of Hormuz, it still needs to ensure the expeditious transit of neutral ships via safe corridors, since there is no effective blockade. 

According to Rule 32 of the San Remo Manual, neutral vessels may exercise the right of non-suspendable innocent passage through belligerent international straits. In addition, its Rule 27 stipulates that: “Transit passage through international straits and passage through waters subject to the right of archipelagic sea lanes passage shall not be impeded unless safe and convenient alternative routes are provided.” But as Heintschel von Heinegg notes (at pp. 267, 270), the airspace above an international strait can be closed for overflight of neutral and belligerent aircraft in an IAC. He (at p. 265) as well as Caminos and Cogliati-Bantz (at p. 30) also find that the strait State might, arguably, be allowed to close navigation in an international strait entirely if ‘justified by the gravest of circumstances’. Arguably, this seems to be the situation that Iran is currently facing.

Nonetheless, absent of an effective blockade, the continued applicability of the right of transit passage in times of armed conflict prima facie means that the suspension of all commercial navigation through the Strait of Hormuz cannot be lawful on the pretext of an on-going naval warfare. However, such a liberal transit regime does not apply in a strait if it is instead governed by the regime of innocent passage. 

The Right of Innocent Passage in the Strait of Hormuz

Notably, Iran claims that the legal regime of innocent passage and not that of transit passage applies in the Strait of Hormuz. The right of (strait-specific) non-suspendable innocent passage (Art 45(2) of UNCLOS) cannot serve the United States’ interests in and over the Strait of Hormuz. Pursuant to Article 20 of the UNCLOS, submarines and other underwater vehicles in non-suspendable innocent passage are required to navigate on the surface and to show their flag. In addition, foreign aircraft enjoy the freedom of overflight in transit passage, which, by comparison, is not applicable for aircraft under the right of non-suspendable innocent passage. Extensive restrictive requirements apply to non-suspendable innocent passage (Art 21 of UNCLOS). Unlike the right of transit passage, the regime of innocent passage enables the coastal State to adopt the necessary steps in its territorial sea to prevent passage which is not innocent (Art 25(1) of UNCLOS).

Iran has not ratified the UNCLOS and the right of transit passage is an innovative concept first introduced under the UNCLOS. Iran considers that the regime of transit passage is not part of customary international law and only States Parties to UNCLOS are entitled to benefit from the right of transit passage. Also, if the new transit regime has entered into force as customary international law, then chances are good that Iran remains unbound due to its status as a persistent objector. Iran has signed the 1958 Convention on the Territorial Sea and the Contiguous Zone as well as the UNCLOS, and the 1969 Vienna Convention on the Law of Treaties, but did not ratify these treaties.

Similar to Iran, the United States as one of the main user States of the Strait of Hormuz and Israel as the other belligerent in the on-going IAC are not States Party to the UNCLOS. But different from Iran, the position of the United States and Israel is that the right of transit passage is part of customary international law. As explained by James Kraska (at p. 229), an argument can be made based on the drafting history of the UNCLOS that Iran cannot benefit from a 12-NM-wide territorial sea without granting free passage of ships and aircraft in and over the Strait of Hormuz.

Based on the literature (compare, e.g., Mahmoudi at p. 348 and Heintschel von Heinegg at p. 266), different from a 12-NM-wide territorial sea and the right of non-suspendable innocent passage, it is not clear if the right of transit passage forms part of customary international law. Hence, for the time being, the passage regimes of the Strait of Hormuz still depend somewhat on the flag State’s status as either a party or a non-party to UNCLOS. Such prominent user States of the Strait of Hormuz as China, Japan, South Korea, the EU Member States, the UK, Norway, and other States Parties to UNCLOS can clearly invoke the applicability of the right of transit passage in the Strait of Hormuz. The same applies in the on-going IAC, unless these States lose their neutrality and become belligerents. 

In my view, the strait States Iran (as a signatory State to UNCLOS) and Oman (as a State Party to UNCLOS) need to respect the right of transit passage of States Parties to UNCLOS in the Strait of Hormuz (Lott 2022, at pp. 166-8). If the right of transit passage does not form part of customary international law, then non-parties to UNCLOS, including the United States and Israel, can at least invoke the customary right of non-suspendable innocent passage for transiting the Strait of Hormuz under the terms of the 1949 Corfu Channel judgment.

Conclusion

There is no (de facto) blockade in the Strait of Hormuz. Presumably, at least some of the attacked merchant vessels in and around the Strait of Hormuz were neutral and civilian objects entitled to the right of transit passage, whereas others might have turned into enemy merchant vessels and military objectives due to their activities. Legally, ships and aircraft of neutral States can continue to exercise their right of transit passage in and over the Strait of Hormuz. The belligerents United States and Israel should be cautious about escorting merchant vessels in a convoy, since this might render the vessels which otherwise need to be treated as civilian objects as Iran’s military objectives.

Unfortunately, the strait States and user States disagree on the passage regime that applies to the Strait of the Hormuz. It results in a parallel legal regime and in a ‘grey area’ in relation to the governance of the Strait of Hormuz. This seems to be the consequence of partial non-applicability of UNCLOS to the regulation of navigation in a strait. This is due to the strait State’s objection to the regime of transit passage as well as many of its user States’ (including the United States’ and Israel’s) decision to abstain from becoming a party to the UNCLOS.