The Iranian Navy's largest vessel sank in the Gulf of Oman after catching fire early June 2 under unknown circumstances, but the entire crew was evacuated safely.
The fire broke out in the engine room of the Kharg, a support vessel, a navy statement said on June 2, without elaborating.
Firefighters battled "for 20 hours" to put out the blaze before the ship went down, it said.
The vessel was located near the Iranian port of Jask, some 1,270 kilometers southeast of Tehran on the Gulf of Oman. The location is near the strategic Strait of Hormuz at the head of the Persian Gulf through which one-fifth of world oil output passes.
"Because of the spread of the fire, the mission to save the Kharg failed and it sank in waters off Jask," the statement said, adding that the vessel had left for a "training mission" in international waters days ago.
The Iranian Navy said that 20 sailors out of a crew of 400 had sustained superficial injuries before being rescued.
No cause was given for the fire.
The Kharg was built in Britain and launched in 1977. It entered the Iranian Navy in 1984.
Iran's navy has been beset by several serious incidents recently.
In April, Iran said one of its vessels, the Saviz freighter, had been targeted in the Red Sea, after media reports said the ship had been attacked with limpet mines. Iranian media reports said Israel had attacked the ship.
During an Iranian military training exercise last year, a missile mistakenly struck a naval vessel near Jask, killing 19 sailors and wounding 15.
In 2018, an Iranian Navy destroyer sank in the Caspian Sea.