The US on Wednesday (March 4) took credit for a submarine attack which sank an Iranian naval frigate off Sri Lanka’s coast. According to Sri Lanka’s deputy foreign minister, at least 80 people were killed after the strike in the Indian Ocean, Reuters reported.

https://t.co/PiqQpVIrMu pic.twitter.com/Wc1e0B0um7

— Department of War 🇺🇸 (@DeptofWar) March 4, 2026

The IRIS Dena was one of the 19 foreign warships that participated in the International Fleet Review conducted in Vishakapatnam on February 17 and 18. It is believed to have been returning from the military exercise when it was struck.

“An American submarine sunk an Iranian warship that thought it was safe in international waters,” US Defense Secrtary Pete Hegseth said. “Instead, it was sunk by a torpedo.” Hegseth called it the first sinking of an enemy ship by torpedo since World War Two.

The IRIS Dena, a Moudge-class frigate belonging to the Iranian Navy’s Southern Fleet, was operating roughly 40 nautical miles off the coastal city of Galle — outside Sri Lankan territory but still within its economic zone —when it sounded a distress call at roughly 5:30 am local time.

The Sri Lankan navy told the BBC that it has rescued 32 people of the 180 people believed to have been on board, although the exact number of missing is unknown.

The sinking of the ship comes as the war between the US and its ally Israel with Iran continues into its fifth day. A coordinated military operation between the US and Israel has targeted Iranian military infrastructure and killed Iran’s Supreme Leader, Ayatollah Ali Khamenei, along with several senior officials. In turn, Iran has struck at US military assets across West Asia.

The International Fleet Review

The International Fleet Review 2026 (IFR 2026) is an international maritime exercise held by India in Vishakapatnam to improve the Indian Navy’s relations with other navies in the region.

Welcome! | خوش آمدید!

🇮🇳🤝🇮🇷 Indian Navy welcomes IRIS Dena, of the Iranian Navy, on her arrival at #Visakhapatnam to participate in #IFR2026_India and #MILAN2026, reflecting long-standing cultural links between the two nations.@India_in_Iran #BridgesOfFriendship… pic.twitter.com/O77v2qNJHJ

— Eastern Naval Command 🇮🇳 (@IN_HQENC) February 17, 2026

According to a press release, 85 ships, including the 19 foreign warships, had participated in the fleet review. The Indian fleet included 60 ships of the Indian Navy, four Indian Coast Guard vessels, and one ship each from the Shipping Corporation of India and the National Institute of Ocean Technology.

Indian President Droupadi Murmu reviewed the Fleet from INS Sumedha, an indigenously built Offshore Patrol Vessel, which served as the Presidential Yacht for the occasion. INS Sumedha was adorned with the Ashoka Emblem and flew the President’s Standard at the mast.

The International Fleet Review has been hosted by different nations, with India hosting the 2001 and 2016 editions.

The latest edition was coupled with the Indian Navy’s mega biennial exercise MILAN, as well as the Indian Ocean Naval Symposium (IONS) Conclave of Chiefs at Visakhapatnam, during which the Indian Navy would assume Chairmanship for the second time (2025-27). The US and Russian navies were originally scheduled to participate in Milan.

About the IRIS Dena

The IRIS Dena was the newest destroyer vessel in Iran’s naval fleet, and was commissioned as part of Iran’s domestically produced Moudge or Mowj class multi-role guided missile frigates in 2021. The Moudge is a class
of domestically-produced Iranian light frigates. These ships are described by Iran as destroyers, but this is a classification that is rejected internationally.

The warship featured surface-to-air missiles, anti-ship missiles, a cannon, machine guns and torpedo launchers.

Dena was Iran’s first destroyer equipped with the Bonyan 4, four domestically produced engines each with 5,000 horsepower, as well as the Bow Truster system to enhance its manoeuvrability.

As part of Iran’s 86th naval fleet, Dena and the IRIS Makran, an oil tanker which was converted into a support ship, had undertaken a 360-degree mission around the planet in 2023, which included a port stop in Brazil despite US pressure. The US Treasury Department included both ships in a sanctions designation in February 2023.

Many questions around the strike

The latest strike off Sri Lanka has raised questions about the legality of a military strike in international waters. A military strike in international waters is typically prohibited under international law, given that international waters are reserved for “peaceful purposes”, under Article 88 of the United Nations Convention on the Law of the Sea (UNCLOS).

A state could use force in international waters if it was responding in self-defence, according to a 2022 article in the Leiden Journal of International Law. Alternatively, the Casebook of the International Committee of the Red Cross (ICRC) cites authorisation by the UN Security Council as grounds for a strike. It is currently unclear if the Dena was participating in the hostilities.

While international maritime law falls under the purview of the UNCLOS, regarded as the Constitution of the Oceans, the US is not a party to it.

The 1982 Convention is a comprehensive international law that lays down the broad frameworks for legitimate behaviour on, and use of, seas and oceans everywhere. It defines the rights and duties of nations regarding activities in the oceans, and also addresses issues such as sovereignty, passage rights, and rights of exclusive economic usage.