Sri Lanka's Foreign Minister Vijitha Herath said the country rescued 32 "critically wounded" sailors aboard the Iranian frigate IRIS Dena, which sank just outside the island's territorial waters, said.

No information was immediately available on the cause of the explosion that sank the vessel, but Herath told parliament that the injured sailors were taken to a hospital in the island's south.

The 180-crew frigate had issued a distress call at dawn.

Sri Lanka's defence ministry said it was searching for the other crew of the IRIS Dena, which went down about 40 kilometres (25 miles) south of the island.

"We are keeping up a search, but we don't know yet what happened to the rest of the crew," an official told AFP, dimming prospects for finding any more survivors.

Herath said two Sri Lankan navy vessels and an aircraft were deployed for the rescue operation, but did not say what caused the Iranian warship to sink.

An opposition legislator asked in parliament whether the vessel had been bombed as part of the ongoing US-Israeli attacks against Iran, but there was no immediate response from the government.

Navy spokesman Buddhika Sampath said their operation was in line with Sri Lanka's maritime obligations and that 32 Iranian sailors were evacuated to the main hospital in Galle, 115 kilometres (70 miles) south of the capital Colombo.

"We responded to the distress call under our international obligations, as this is within our search and rescue area in the Indian Ocean," Sampath told AFP.

Both Sri Lanka's navy and the air force said they were not releasing footage of the rescue because it involved the military of another state.

Police stepped up security outside the Galle hospital as the wounded Iranians were brought there by the local navy.

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