Russia’s shadow fleet is an increasingly serious challenge for European maritime security, extending beyond the evasion of sanctions on Russian oil exports. Aging and inadequately insured vessels, opaque ownership and registration, and, in some cases, uncertain nationality create safety and environmental risks, while suspected involvement in damage to critical offshore infrastructure has exposed gaps in enforcement. Russia’s war against Ukraine adds a further dimension, raising questions under the law of naval warfare, including when merchant vessels may lose the protections afforded to civilian objects.
The study finds that there is no single legal basis for restricting the shadow fleet. Instead, international law offers a range of complementary avenues depending on the circumstances of each case. What states can do depends on a vessel’s nationality and conduct, its location and applicable passage regime, the conduct of its flag state, and, during armed conflict, whether it qualifies as a military objective. The study identifies opportunities for stronger action against vessels with uncertain nationality, location-specific enforcement options in international straits, measures responding to environmental risks, countermeasures against flag states, and, in armed conflict, action against individual vessels that meet the requirements for military-objective status.
Several concrete avenues for stronger maritime law enforcement exist, particularly where vessels have uncertain nationality. Under Article 110(1)(d) of the UN Convention on the Law of the Sea (UNCLOS), states may board vessels on the high seas or in an exclusive economic zone where there are reasonable grounds for suspecting that they are stateless in order to verify their nationality. The report recommends more systematic use of this provision where a vessel’s claimed nationality cannot be affirmatively confirmed by its flag state. Once nationality is established, however, further action generally requires an additional jurisdictional basis.
International straits present important—but highly location-specific—opportunities for enforcement. The report argues that the Danish Straits are governed by the special regime of non-suspendable innocent passage under the 1857 Copenhagen Convention, leaving Denmark greater scope to act against vessels engaged in non-innocent passage. Abolishing the EEZ corridor in the Viro Strait could strengthen Estonia’s and Finland’s ability to protect critical underwater infrastructure, but would also expand transit-passage rights for foreign ships and aircraft across much of the Gulf of Finland. In the Strait of Dover, Article 233 of UNCLOS provides a potential basis for enforcement where violations cause or threaten major environmental damage, although restrictions there could simply divert shadow-fleet traffic around the British Isles.
Threats to critical offshore infrastructure create additional potential grounds for state action. The report explores a dynamic interpretation of the piracy provisions of UNCLOS under which deliberate damage to seabed infrastructure could constitute piracy ” potentially providing universal jurisdiction to interdict and arrest vessels regardless of flag. It also identifies Article 113 of UNCLOS, necessity under the law of state responsibility, and the Suppression of Unlawful Acts Convention as complementary legal bases that may support action to protect submarine cables and pipelines.
International environmental obligations may also provide a basis for stronger measures. The customary duty of prevention and the associated due-diligence standard require states not merely to adopt safety and environmental rules, but to ensure their effective implementation and enforcement. These obligations may provide a basis for affected coastal states to limit access by shadow tankers where they pose heightened risks of serious environmental damage. Coastal states could also consider restrictions based on defined safety, insurance, and environmental standards while seeking their incorporation into international rules through the International Maritime Organization (IMO).
Systematic failures by flag states to fulfill their international obligations may open an additional avenue through the law of countermeasures. Persistent failures to exercise effective jurisdiction and control over vessels and enforce applicable safety and environmental requirements could justify temporary restrictions on passage, provided that the requirements governing countermeasures are met, including proportionality, reversibility, prior notification, and an opportunity for the responsible state to comply. Russia presents an additional case as it increasingly serves directly as a flag state for shadow-fleet vessels. Alongside breaches of flag-state obligations, the report examines the prohibition of aggression as a separate basis for measures against Russia and proposes increasing institutional pressure through the IMO, including by seeking the suspension of Russia’s participation or voting rights in the IMO Assembly and relevant committees.
Shadow fleet affiliation alone is not enough to classify a merchant vessel as a military objective. The report concludes that the law of blockade did not provide a legal basis for Russia’s attacks and threats against neutral merchant vessels in the Black Sea and that its 2023 threat to treat vessels bound for Ukrainian ports as military targets was inconsistent with the law of naval warfare.
Individual shadow-fleet vessels may nevertheless become military objectives under certain circumstances, although the applicable legal bases differ. Some states consider civilian objects that generate revenue supporting a belligerent’s war effort capable of becoming military objectives, but this “war-sustaining” doctrine is not generally accepted. The report notes that Ukraine’s attacks on shadow-fleet tankers may signal support for this doctrine, while recognizing that the interpretation remains contested. Irrespective of that debate, individual merchant vessels may become military objectives through direct participation in military action, for example by carrying troops or military material, transporting oil for direct military use, contributing to intelligence collection, or operating under the control of the enemy government.
Even where a vessel qualifies as a military objective, legal constraints continue to apply, including precautions, proportionality, and due regard for environmental damage. The report notes that Ukraine’s reported attacks have so far targeted tankers on their way to Russian oil export terminals and were therefore presumably empty. It also proposes considering protected zones in particularly sensitive semi-enclosed seas, including the Baltic and Mediterranean.
Taken together, the study points toward a differentiated rather than uniform response, tailored to the vessel, its location and conduct, and the applicable legal framework.
