Sri Lanka rescued 32 “critically wounded” sailors aboard the Iranian frigate IRIS Dena, which sank on Wednesday just outside the island’s territorial waters, Foreign Minister Vijitha Herath 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 40km (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 said, 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.