Sovereign Immunity at Sea Under UNCLOS: Meaning, Boundaries, and Operational Use

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Created: 06 May 2025

Introduction

“Sovereign immunity at sea” is a vessel-focused rule: warships and State-owned/operated ships used only for government non-commercial service remain protected from the jurisdiction of any State other than the flag State, even when operating near another State’s coast. The doctrine is operationally decisive because it controls what actions are legally unavailable—especially boarding, arrest, detention, seizure, or judicial process—when the platform is sovereign immune.1

UNCLOS pairs that protection with a restrictive compliance model. For warships in the territorial sea, the coastal State’s principal immediate remedy is to require departure when laws and regulations are not complied with, while responsibility for loss or damage caused by the warship’s non-compliance rests at the State-to-State level. The doctrine preserves sovereign equality while keeping tactical activity inside lawful limits.2

1. Why This Matters

Operational risk concentrates in the first minutes of contact. A mistaken enforcement action against a sovereign immune vessel—such as physical restraint, attempted boarding, or attachment—can convert a manageable incident into a legal wrong with immediate diplomatic and security consequences. A stable doctrine, applied early, reduces escalation and keeps the response defensible: identify sovereign status, avoid measures of constraint, and preserve options for State-to-State follow-on action based on documented facts rather than coercive measures. The ITLOS treatment of warship immunity in port underscores that interference with mission discharge implicates immunity and can endanger friendly relations among States.3

2. The Core Rule

Under UNCLOS and general international law, sovereign immunity means foreign warships and non-commercial government ships are not subject to another State’s jurisdiction, and enforcement measures against them are legally constrained. In internal waters and ports, entry is governed by the coastal State’s consent and conditions, but immunity still blocks detention or attachment as a response tool once a sovereign immune platform is present. In the territorial sea, the coastal State may require a non-compliant warship to leave, but coercive enforcement against the ship is not the default legal pathway. In the EEZ (and beyond), complete immunity applies by UNCLOS structure, reinforcing a restrictive approach: document, deconflict, and elevate to government-level engagement rather than compel through force.4

3. Definition and Scope

Sovereign immunity at sea is a status-based protection attached to specific State platforms—warships and qualifying non-commercial government ships—shielding them from foreign jurisdiction and from enforcement measures that would otherwise be available against private vessels. It operates alongside, not in place of, navigational regimes; passage rights answer where a ship may operate, while sovereign immunity answers what a coastal/port State may lawfully do to the platform when it is present.5

Sovereign immunity at sea is not a general exemption from law, and it is not a blanket defense for every State-connected vessel. It is not immunity for private merchant ships, not immunity for State-owned ships engaged in commercial service, not a license to disregard coastal State safety requirements, and not a substitute for responsibility and State-to-State remedies when harm occurs.6

4. Doctrine: Elements and Tests
  1. Platform Qualification (Status Test): Confirm whether the platform is a warship under UNCLOS or a State-owned/operated ship used only for government non-commercial service.
    Operational implication: Status controls the response menu; once sovereign immunity plausibly applies, enforcement measures become legally high-risk and should default to non-coercive management.7
  2. Zone Identification (Where the Interaction Occurs): Fix the maritime zone as internal waters/port, territorial sea, EEZ, or high seas for the interaction in question.
    Operational implication: Zone affects available coastal State powers, but it does not erase immunity for sovereign immune platforms; the lawful response remains restrictive even when local jurisdiction is otherwise robust.8
  3. Action Characterization (Constraint vs. Non-Constraint): Distinguish between non-coercive actions (hail, shadow, record, communicate, coordinate) and measures of constraint (board, arrest/detain, seize/attach, compel movement, initiate local criminal process against the platform).
    Operational implication: Non-coercive actions preserve safety and legal defensibility; measures of constraint against sovereign immune platforms are the primary error pathway to an internationally wrongful act.9 10
  4. Territorial Sea Remedy for Warships (Departure Model): If a warship does not comply with coastal State laws/regulations in the territorial sea, the principal immediate remedy is to require the warship to leave.
    Operational implication: Manage the event toward safe exit and documentation rather than enforcement; shift follow-on to flag-State engagement and, where applicable, State responsibility mechanisms.11
  5. Ports and Internal Waters Guardrails (Consent/Entry Control): Internal waters and ports fall under coastal State sovereignty, and port entry/continued stay is controlled through permission and conditions. Immunity still blocks attachment or detention as an enforcement substitute once a warship is present.
    Operational implication: Use consent tools (deny entry, set conditions, withdraw permission, request departure) and reserve coercive measures for clearly lawful bases that do not conflict with sovereign immunity.12 13
5. Operational Application

Treat every interaction with a suspected sovereign immune platform as a two-track problem: (1) immediate safety and deconfliction through non-coercive control, and (2) legal positioning through disciplined documentation for government-level follow-on action.14 15

Warship Conduct in the Territorial Sea: Warships may be identified, monitored, and routed or escorted, provided these measures remain non-coercive, and a departure directive may be issued if non-compliance continues. Coercive actions such as boarding, arrest, detention, seizure, or compelled movement are prohibited. Documentation should include time-stamped track data, visual identifiers and external marks, communications logs, applicable coastal State regulations, and decision records for any departure directives and safety measures.16

Government Non-Commercial Ship Operating in the EEZ: Non-commercial government ships may be subject to verification steps, operational coordination, and recording of activities affecting coastal State rights while ensuring safety. Measures of constraint based solely on domestic jurisdiction are not permitted when the vessel qualifies for complete immunity. Documentation should include evidence of State ownership or operation in a non-commercial capacity, flag and markings, behavior relevant to the EEZ context, chain-of-command approvals, and contemporaneous imagery or sensor logs.17

Warship in Port or Internal Waters: Entry and stay may be conditioned on consent, and protective port safety measures may be applied without creating coercive constraint. Formal requests for departure may be issued, but attachment, detention, or seizure to enforce claims or judgments is prohibited. Documentation should include clearance or permission records, terms and conditions communicated, any safety or security incidents, actions taken to avoid constraint, and diplomatic notifications or handover packages for lawful State-to-State handling.18

6. Case Illustrations

A. The “ARA Libertad” (Argentina v. Ghana): Warship Immunity in Port

The Argentine Navy training frigate ARA Libertad entered the port of Tema, Ghana pursuant to prior diplomatic arrangements and consent for a scheduled visit. While in port, Ghanaian authorities detained the vessel pursuant to a domestic court process and impeded its departure, with reported attempts by port authorities to board and reposition the ship by force, and with related domestic proceedings directed at the ship’s commander. Argentina then initiated Annex VII arbitration under UNCLOS and sought provisional measures from ITLOS pending constitution of the arbitral tribunal.19

ITLOS treated the warship as an expression of the flag State’s sovereignty and accepted that, under general international law, a warship enjoys immunity, including in internal waters. The Tribunal emphasized the sensitivity of acts that prevent a warship from discharging its mission and duties, and it prescribed provisional measures requiring Ghana to release the warship unconditionally and allow it to leave. Operationally, the case fixes a bright boundary: port presence does not convert a warship into attachable property, and legal disputes are not lawfully managed through measures of constraint against the platform.20

B. Detention of Three Ukrainian Naval Vessels (Ukraine v. Russian Federation): Immunity and Release as Provisional Measures

On 25 November 2018, three Ukrainian naval vessels (two artillery boats and a naval tug) and their crews were arrested and detained by Russian authorities following an incident in the Black Sea near the Kerch Strait, after communications asserting closure of the strait, physical blocking, pursuit, and subsequent seizure and detention of the vessels and servicemen, with domestic criminal proceedings referenced by the detaining State. Ukraine then instituted Annex VII proceedings and requested provisional measures from ITLOS pending constitution of the arbitral tribunal.21

At the provisional stage, ITLOS accepted that the vessels’ status as warships (and a naval auxiliary) was not disputed and treated the claimed immunity rights as plausible under the UNCLOS structure and general international law. The Tribunal prescribed provisional measures requiring immediate release and return of the vessels and servicemen, treating continued detention as capable of irreparably prejudicing immunity-based rights. Operationally, the case reinforces the same discipline as ARA Libertad: once sovereign immunity is engaged, detention and comparable measures of constraint sit at the center of legal exposure, and the safer posture is controlled deconfliction plus government-level resolution.22

Conclusion

Sovereign immunity at sea can be applied using a clear operational principle: any contemplated action must be achievable without asserting foreign jurisdiction over the vessel or imposing coercive measures. If this is not possible, the action falls outside the restrictive posture and should instead rely on non-coercive measures and State-to-State communication. UNCLOS supports this approach through provisions on warship status, immunity, complete immunity, and the model for warships departing territorial seas, while ITLOS practice confirms that presence in ports or internal waters does not nullify immunity and that coercive measures against warships are legally problematic. Key watchpoints include correctly classifying the vessel, understanding the maritime zone, and avoiding instinctive enforcement in ports. A defensible approach relies on early determination of vessel status, careful documentation, and lawful exit or consent management, rather than physical compulsion.23 24 25

Footnotes

  1. United Nations Convention on the Law of the Sea, Dec. 10, 1982, 1833 U.N.T.S. 3, arts. 29, 32, 95–96 [hereinafter UNCLOS]. ↩︎
  2. UNCLOS arts. 30–31. ↩︎
  3. The “ARA Libertad” Case (Argentina v. Ghana), Provisional Measures, Order, ITLOS Case No. 20, ¶¶ 93–98 (Dec. 15, 2012) (hereinafter ARA Libertad). ↩︎
  4. UNCLOS arts. 2, 8, 30, 58(2), 95–96. ↩︎
  5. Id. arts. 17, 29, 32, 95–96, 96. ↩︎
  6. Id. arts. 30–31, 96. ↩︎
  7. Id. arts. 29, 96. ↩︎
  8. Id. arts. 2, 8, 30–32, 58(2), 95–96. ↩︎
  9. ARA Libertad ¶¶ 94–100. ↩︎
  10. Case Concerning the Detention of Three Ukrainian Naval Vessels (Ukraine v. Russian Federation), Provisional Measures, Order, ITLOS Case No. 26, ¶¶ 92–94 (May 25, 2019) (hereinafter Detention of Three Ukrainian Naval Vessels). ↩︎
  11. UNCLOS art. 30. ↩︎
  12. Id. arts. 2, 8. ↩︎
  13. ARA Libertad ¶¶ 94–100, 108(1). ↩︎
  14. UNCLOS arts. 30–32, 95–96 ↩︎
  15. ARA Libertad ¶¶ 94–100. ↩︎
  16. UNCLOS arts. 30–32. ↩︎
  17. Id. arts. 58(2), 95–96, 96. ↩︎
  18. ARA Libertad ¶¶ 94–100, 108(1). ↩︎
  19. Id. ¶¶ 38–42, 76, 78, 83, 92. ↩︎
  20. Id. ¶¶ 94–100, 108(1). ↩︎
  21. Detention of Three Ukrainian Naval Vessels ¶¶ 30–32. ↩︎
  22. Id. ¶¶ 92–94, 110–111, 124(1)(a). ↩︎
  23. UNCLOS arts. 29–32, 58(2), 95–96. ↩︎
  24. ARA Libertad ¶¶ 94–100, 108(1). ↩︎
  25. Detention of Three Ukrainian Naval Vessels ¶¶ 92–94, 110–111, 124(1)(a). ↩︎

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