Iran's largest navy ship the Kharg sank on Wednesday after catching fire in the Gulf of Oman, but the crew were safely rescued, Iranian media reported.
No further explanation was given for the latest incident in a region of sensitive waterways, where there have been accusations of attacks on ships owned by arch-enemies Iran and Israel.
State TV said the fire on Iran's highest-tonnage naval vessel started around 2:25 a.m.on Wednesday (21:55 GMT) near the Iranian port of Jask, where it was on a training mission.
The Gulf of Oman connects to the Strait of Hormuz where about a fifth of the world's oil passes.
Rescue operations for the Kharg went on for hours, the statement on state TV said, with all the crew disembarking.
"All efforts to save the vessel were unsuccessful and it sank," the semi-official Fars news agency reported.
In April, Iran said one of its vessels, the Saviz, had been targeted in the Red Sea, after media reports the ship had been attacked with limpet mines.
Source: National News Agency