Explained: What is IRIS Dena and how the US-Iran conflict reached the Indian Ocean

The sinking of an Iranian naval vessel near Sri Lanka has dramatically widened the ongoing confrontation between the United States and Iran. The incident, which reportedly involved a US submarine torpedo attack, has raised serious questions about the expansion of the conflict beyond the Gulf region and into the wider Indian Ocean.

The Federal explains what happened and why it matters.

What exactly happened?

A submarine belonging to the United States sank an Iranian warship off the southern coast of Sri Lanka in the Indian Ocean.

According to Sri Lankan authorities, the vessel was the Iranian frigate IRIS Dena (Islamic Republic of Iran Ship, the commonly-used prefix for Iranian naval vessels), which was returning to Iran after visiting an eastern Indian port.

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The attack reportedly involved a torpedo strike launched from a US submarine. US Defence Secretary Pete Hegseth said the ship was hit while sailing in international waters and described the strike as a sudden and decisive attack.

The significance is also in the rarity of a warship being torpedoed and sunk by submarines. This is the fourth such case, and the first involving the US, since World War 2.

How many casualties were reported?

Authorities in the Sri Lankan port city of Galle said 87 bodies were brought to hospital after rescue operations.

Officials also said that 32 sailors were rescued, and around 60 crew members are still missing. The warship was believed to have had about 180 people on board.

Rescue teams responding to the distress call reported seeing survivors floating in the water before they were picked up.

How did Sri Lanka respond?

Although the attack occurred outside Sri Lankan territorial waters, the Sri Lankan navy launched a search and rescue mission after receiving the distress call.

Sri Lankan naval boats that reached the site reported finding an oil slick and survivors in the sea. Bodies recovered from the water were transported to Karapitiya Hospital in Galle.

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Naval officials said the survivors, including the ship’s commander and senior officers, told rescuers that their vessel had been struck by a submarine-launched torpedo.

Why was the ship in the Indian Ocean?

The frigate had recently taken part in the Milan multilateral naval exercise, a biennial naval drill organised by the Indian Navy.

The exercise took place in the Bay of Bengal near Visakhapatnam on India’s eastern coast from February 18 to 25.

Images posted earlier by the Indian Navy’s Eastern Naval Command showed the Iranian warship arriving in Visakhapatnam for the exercise, which involved several international navies.

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 warship featured anti-ship missiles, surface-to-air missiles, torpedo launchers, machine guns, and a cannon. The vessel, named after Mount Dena in southwestern Iran, also had a helicopter landing pad and radar systems capable of tracking multiple targets.

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It was Iran’s first destroyer equipped with the domestically-manufactured Bonyan 4 engines, each with 5,000 horsepower, and also the Bow Truster system to enhance its manoeuvrability.

The warship had been sanctioned by the US Treasury Department in February 2023.

What are the reactions in India?

Former Navy Chief Admiral Arun Prakash wrote on X, “Sinking of Iranian warship, off southern tip of Sri Lanka, with heavy loss of life is a senseless & inflammatory act. Initiating another dimension of violence in this open-ended conflict will spread alarm across the high seas and disrupt global seaborne commerce. Condemnable!”

Admiral Prakash also said, “The sinking of the Iranian warship shows the Iran-US-Israel war has come to our doorstep. India should convey its deep concern and displeasure to the US.”

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Congress leader Pawan Khera, in a post on X, said, "Today, an Iranian naval vessel - returning from the Milan 2026 International Fleet Review, where it had been invited by India - was sunk by a US submarine at the edge of Indian waters near Sri Lanka. Does India have no influence left in its own neighbourhood? Or has that space also been quietly ceded to Washington and Tel Aviv?"

How does this affect the wider conflict?

The attack marks a significant geographic expansion of the confrontation between the United States and Iran.

Until now, most military activity linked to the conflict had been concentrated around the Gulf region. The sinking of an Iranian naval vessel in the Indian Ocean suggests that the confrontation is spreading into new maritime zones.

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That could increase risks for international shipping routes and naval movements across one of the world’s busiest ocean corridors.