At least 87 people have been confirmed dead after a US submarine struck and sunk IRIS Dena, an Iranian frigate, off the coast of Sri Lanka on Wednesday. Many others remain unaccounted for. According to reports from the Sri Lankan Navy, they managed to rescue 32 Iranian sailors from the frigate. However, about 60 sailors remain unaccounted for.

The incident has drawn attention because the vessel had recently taken part in Milan 2026, a major multilateral maritime exercise hosted by the Indian Navy.

IRIS Dena and its recent deployment

IRIS Dena was a 1,500-ton Moudge-class frigate that was launched in 2015 and was commissioned in 2021 by Iran’s Navy. The frigate was equipped with four Iran-made “Bonyan 4” engines and a vertical launching system.

The warship was in the Bay of Bengal participating in Milan 2026 and was on its way back to Iran when it was attacked. It had also taken part in the exercise in 2024, marking its second appearance in a multinational exercise.

What is Milan 2026?

Milan is the Indian Navy’s flagship biennial multilateral maritime exercise conducted by the Indian Navy to enhance maritime cooperation between India and other friendly nations.

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The 13th edition of the exercise was held from February 21 to February 25 under the theme “Camaraderie, Cooperation, Collaboration.” Hosted in Visakhapatnam, the exercise saw participation from 74 countries, making it the largest edition of Milan so far.

According to the Indian Navy, “Held under the theme 'Camaraderie, Cooperation, Collaboration', the Milan 2026 witnessed participation on an unprecedented scale, comprising 42 ships and submarines and 29 aircraft. These also included 18 ships from the participating friendly foreign countries.”

The event concluded with a closing ceremony onboard India’s indigenous aircraft carrier INS Vikrant off the coast of Visakhapatnam.

Harbour phase and sea phase exercises

The exercise was conducted in two stages. The harbour phase included bilateral interactions, professional exchanges and an international maritime seminar.

This was followed by the sea phase, which featured high-intensity operational drills. These included integrated air defence, anti-submarine warfare exercises, maritime interdiction operations, communication drills and cross-deck flying operations.

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According to the Indian Navy, the objective of Milan is to help participating navies operate together more effectively while addressing emerging maritime challenges. The exercise also focuses on cooperation against non-traditional threats such as piracy and drone swarms.

What the US said about the strike

While announcing the strike, Chairman of the Joint Chiefs Gen Dan Caine said the frigate was sunk using an MK-48 heavyweight torpedo.

He stated that since Saturday, when the US-Israel operation “Epic Fury” began, US forces had destroyed more than 20 Iranian naval vessels, along with a submarine.

“We’ve destroyed more than 20 Iranian naval vessels, in addition to the frigate outside the area, one submarine and effectively neutralised at this point in time Iran’s major naval presence in theater out there,” Caine said.