Beyond the Blockaded Area: Interdicting Iranian Oil and the Limits of Existing Maritime Legal Frameworks

Recent efforts by the United States to interdict Iranian oil shipments in distant waters have extended well beyond the Persian Gulf. These operations have included the interception of tankers carrying Iranian oil—often operating under neutral flags—in areas such as the Indian Ocean, far from any declared blockaded zone. In at least some instances, so-called “sanctioned” vessels have been boarded on the basis that they were transporting Iranian oil. For example, in April 2026, U.S. forces reportedly seized the tanker Tifani in the Indian Ocean, after it had loaded oil from Kharg Island, Iran’s principal export terminal, and departed prior to the announcement of a U.S. blockade. The interception took place in open waters far from the blockaded area. U.S. statements further suggest that interdiction efforts may extend beyond the immediate theatre of war.

A defining feature of the vessels at issue is that they are often not Iranian-flagged, but operate under neutral flags, frequently registered in third States through complex ownership structures. Formally, based on the law of naval warfare, these vessels qualify as neutral merchant ships. At the same time, they may be economically linked to Iranian interests through the transport of oil originating from Iranian ports that in turn provide revenue for Iran which can be used to finance Iran’s war effort.

These operations are often framed by the United States as part of sanctions enforcement. However, sanctions as such do not, in themselves, provide a general legal basis for boarding or seizing foreign vessels on the high seas. This is because sanctions, as unilateral or domestic measures, do not in themselves confer enforcement jurisdiction over foreign vessels operating on the high seas. Under the law of the sea, jurisdiction over such vessels remains primarily with the flag State, and exceptions permitting interference are limited and well-defined. Sanctions, without more, do not fall within those recognised exceptions.

The question, then, is straightforward: are such interdictions and seizures lawful under international law? While the practices observed draw on elements associated with both naval warfare and peacetime enforcement, they do not fit neatly within either framework. At the same time, the way in which the United States frames these interdictions suggests a possible shift in underlying logic: from doctrines structured around the destination of goods to a focus on their economic function, in particular their contribution to a State’s war-sustaining capacity. Whether existing legal doctrines can accommodate such a shift—or whether these practices lack grounding in international law altogether—is the question this post examines.

The Limits of the Law of Naval Warfare

The law of naval warfare provides the most developed framework for the interdiction of maritime commerce during armed conflict. Yet its classical doctrines do not readily apply to the present scenario.

First, blockade is premised on the existence of a defined blockaded area and the concept of breach of blockade. A vessel becomes liable to capture when it attempts to enter or leave such an area in violation of the blockade. Neutral vessels operating in distant waters—far from Iranian ports—cannot easily be characterised as breaching a blockade.

Second, one of the blockade doctrines of continuous voyage allows interception of goods ultimately destined for the enemy, even if routed indirectly. However, this doctrine is historically limited to inbound traffic—the supply of goods to the enemy. Iranian oil shipments, by contrast, are outbound, moving away from the enemy. Extending continuous voyage to such flows would represent a significant doctrinal shift.

Third, contraband regime also depends fundamentally on the destination of goods. As reflected in instruments such as the Paris Declaration of 1856, the San Remo Manual para. 148 and the Newport Manual (9.6.2.1), goods become liable to capture when they are of use to the enemy and ultimately destined for it. The US indeed includes crude oil in the list of “conditional contraband.” However, this framework applies only to vessels destined for the enemy. On the other hand, Iranian oil exports do not meet this criterion: they are not destined for the enemy, but rather involve goods moving outward from Iran.

In combination, these doctrines suggest that classical law of naval warfare—structured around destination-based control—does not comfortably extend to the interdiction of outbound resource flows in distant waters.

Law of the Sea and the Limits of Peacetime Enforcement

If the law of naval warfare does not provide a clear basis, the question arises whether such operations can be justified under the law of the sea. And indeed, both bodies of law may also apply at the same time.

Under peacetime rules, jurisdiction over vessels on the high seas rests primarily with the flag State. Coercive measures such as boarding are permitted only under limited exceptions, including piracy, slave trade, statelessness, or with the consent of the flag State.

U.S. statements have, in some cases, characterised targeted vessels as stateless, invoking the right of visit. Where such classification is factually accurate, it provides a recognised legal basis for boarding. Where a vessel is treated as stateless, it may open the possibility for the exercise of jurisdiction by the intercepting State. This may, in turn, create a basis for invoking domestic legal frameworks—such as sanctions enforcement or forfeiture proceedings—subject to the existence of a sufficient jurisdictional nexus.

However, this move from boarding to enforcement does not resolve the underlying doctrinal difficulty. While statelessness may justify the initial act of boarding under the law of the sea, it does not in itself provide a sufficient legal basis for subsequent enforcement measures, particularly where these are directed at conduct that is not subject to universal jurisdiction, such as unilateral sanctions violations. Not all neutral merchant vessels associated with the transport of Iranian oil, moreover, can be readily characterised as stateless; some may operate under formal registrations, albeit within complex and opaque ownership arrangements.

Earlier U.S. practice, such as United States v. Suez Rajan, illustrate the use of domestic forfeiture proceedings following the establishment of control over cargo or vessels. However, such cases do not in themselves provide a legal basis for the initial interdiction at sea. Treating statelessness as a gateway to full enforcement jurisdiction risks conflating the limited right of boarding with a broader, and less clearly grounded, claim to regulate foreign vessels on the high seas under domestic law.

Moreover, even where boarding may be justified under the law of the sea, this framework does not readily support the broader practice of targeting vessels on the basis of their economic connection to a sanctioned State. This difficulty is reinforced by the classical law of naval warfare, as reflected in the Paris Declaration of 1856, which sought to protect neutral commerce from undue interference. In particular, the principle that neutral flags cover enemy goods—subject to the exception of contraband, which, again, depends on goods being destined for the enemy, illustrates a broader commitment to preserving the flow of international trade, even in times of conflict. Against this background, extending interdiction to vessels on the basis of their economic linkage to a sanctioned State risks moving beyond the traditional limits imposed on interference with neutral shipping.

Neutral Vessels and the Question of “Enemy Character”

The involvement of neutral-flagged vessels further complicates the analysis. The London Declaration of 1909, although not formally ratified, recognises that a vessel’s enemy character may depend on factors beyond its flag, including ownership, control, and other relevant criteria.  

In classical prize law, vessels flying an enemy flag are prima facie liable to visit, search, and capture during armed conflict. Neutral-flagged vessels, by contrast, are generally protected, but may also become subject to visit, search, and capture where they are found to possess an enemy character

However, extending this concept to vessels engaged in the commercial transport of oil raises difficult questions. Many such vessels are not in the exclusive employment of the Iranian government, nor are they controlled by the government of Iran, nor do they otherwise exhibit recognised indicators of enemy character. While they may be economically linked to Iranian interests, this alone does not clearly establish enemy character within the traditional framework.

As a result, reliance on enemy character in this context risks stretching the concept beyond its established limits.

Between Regimes: Toward a Functional Logic?

What emerges from the foregoing analysis is not a clear legal prohibition, but a structural mismatch. The interdiction practices observed do not fit neatly within either law of naval warfare’s visit, search, and capture or law of the sea’s enforcement.

Instead, they appear to reflect a hybrid approach: drawing on elements of sanctions policy of domestic law, invoking selected legal hooks such as statelessness of the law of the sea, and employing forms of maritime interdiction traditionally associated with wartime operations, including the use of “visit” terminology and boarding operations conducted by naval forces. While such boarding practices are not unknown in peacetime, their application in this context—targeting neutral vessels on the basis of economic linkage—marks a significant departure from established patterns of maritime enforcement. U.S. statements further suggest a broader framing of these measures, indicating that interdictions may extend beyond the immediate theatre and, at least in narrative terms, resemble an expanded form of blockade-type enforcement.

At the same time, the underlying logic of these operations appears to shift from destination-based reasoning toward a focus on economic function. Rather than regulating access to enemy territory or the supply of goods to the enemy, the practice targets outward flows of resources on the basis of their contribution to the enemy’s war-sustaining capacity.

Whether this represents an emerging practice, or remains limited to a particular State’s approach, remains an open question. 

What remains unclear, however, is not only how such practices are to be classified, but also whether they can be justified under existing legal frameworks at all.

Implications for the Use of Force at Sea

One implication of this uncertainty concerns the applicable standards governing the use of force at sea. Interdictions of neutral-flagged tankers operating outside any blockaded area, and carrying oil from Iran to neutral ports, cannot readily be justified under the law of naval warfare. In the absence of a clear wartime framework, such operations are more appropriately assessed against the standards governing maritime law enforcement in peacetime.

This distinction matters in practice. Unlike blockade enforcement or contraband capture—where different rules may apply in armed conflict—law enforcement at sea has more restrictive standards that are governed by principles of necessity, reasonableness, and proportionality.

Conclusion

The interdiction of Iranian oil shipments in distant waters does not fit comfortably within existing maritime legal frameworks. Classical doctrines of the law of naval warfare do not extend easily to outbound resource flows, while peacetime law of the sea enforcement provides only limited bases for interference with neutral shipping. What emerges is a practice that draws selectively from different regimes without being fully grounded in any of them. The question, therefore, is not only how such practices should be characterised, but whether they can be justified under international law at all.