What are the legal processes and precedents for the U.S. seizing foreign-flagged tankers carrying sanctioned cargo?
Executive summary
The United States can and has seized foreign-flagged tankers carrying cargo it deems sanctionable, but those actions rest on a mixed legal toolkit—domestic warrants and forfeiture laws, cooperation with flag or third-party coastal states, and pressure on insurers/owners—rather than an unfettered right to board and take vessels on the high seas [1] [2]. Recent cases such as the Grace 1, the Bella-related cargo seizures and the December 2025 interdictions around Venezuela (Skipper/Centuries) illustrate both the options Washington uses and the clear international-law limits and controversies such seizures generate [1] [3] [4].
1. The international legal baseline: flag-state primacy and UNCLOS limits
Under accepted international maritime law the flag state has primary jurisdiction over a vessel on the high seas, and a third state generally cannot unilaterally board and enforce its domestic law absent a specific treaty exception—an interpretation emphasized by the International Tribunal for the Law of the Sea and by commentators who say unilateral boarding for sanctions enforcement “has no standing in international law” [5] [6].
2. U.S. domestic authorities and forfeiture statutes as the operational lever
Within U.S. law, courts can issue arrest/seizure warrants and initiate forfeiture of cargo or vessels linked to sanctions evasion or criminal activity, and federal agencies (FBI, DHS, Coast Guard) can execute such warrants with military support where needed, creating a domestic legal basis for taking custody of vessels or oil cargoes once they are in U.S. control or declared subject to U.S. forfeiture processes [3] [7] [2].
3. Cooperative and coercive paths: flag-state consent, third-party enforcement, or commercial pressure
When the flag state cooperates, seizures are straightforward; when it does not, the U.S. has relied on third-party coastal enforcement in territorial waters (as with Grace 1) or on convincing insurers, shipowners, managers and port authorities to surrender cargo or comply under the threat of secondary sanctions and denial of services—measures that effectively coerce compliance without a high-seas boarding by U.S. forces [1] [8].
4. Precedents: Grace 1, Bella cargo seizures, and the Venezuela interdictions
The 2019 Grace 1 stoppage (involving third-party coastal state enforcement) and the later court-ordered cargo seizures of the Bella family of tankers show the U.S. carefully using allied authorities and judicial mechanisms where possible, while the 2025 Skipper and Centuries operations demonstrate a more aggressive posture combining a U.S. seizure warrant, military/Coast Guard boarding and subsequent U.S. custody of cargo and vessel pending forfeiture—actions that generated rapid international dispute and claims of illegality [1] [3] [4].
5. Legal gray zones and contested legality
Scholars and international commentators stress that boarding a foreign-flagged ship on the high seas to enforce unilateral sanctions is legally fraught: the Conversation and legal analyses argue the U.S. rationale may lack an unambiguous basis under international law unless the vessel was stateless, falsely flagged, or the flag state expressly consented, and critics say the U.S. has not always publicly squared its actions with the specific UNCLOS exceptions [5] [4] [6].
6. Evidentiary and procedural steps after capture: delisting, forfeiture, and ownership challenges
Operationally, the U.S. seeks judicial warrants grounded in sanctions designations or criminal predicates, relies on evidence such as spoofing, false flags or ties to designated networks, and then proceeds through forfeiture and maritime-adjudication processes—steps that can be complicated if a flag state “de-lists” a vessel or if ownership is hidden behind shell companies, prompting long legal battles over custody and title [4] [9] [2].
7. Geopolitical calculus and hidden agendas shaping enforcement choices
Seizures are not purely legal acts; they are strategic signals that can escalate tensions with states whose vessels or clients are targeted, and they also reflect domestic political imperatives to demonstrate enforcement—observers note rhetoric about “narco‑terrorism” or “shadow fleets” can be used to justify forceful interdictions while also advancing broader foreign-policy aims [10] [11] [12].
Conclusion
The United States has several lawful tools to interdict sanctioned cargo—judicial warrants, cooperation with flag/third states, and economic pressures on maritime service providers—but using them against foreign-flagged vessels on the high seas runs into clear international-law constraints, factual thresholds (false flag/statelessness or flag consent), and likely diplomatic fallout; the recent mix of court-ordered seizures, coercive commercial measures and military-assisted boardings illustrates both how Washington navigates those limits and why the legality of some high-seas seizures remains contested in international fora [1] [3] [5] [4].