Iran's army said on Monday that 19 people were killed in an "accident" involving a warship hit by a friendly fire missile during exercises in the Gulf of Oman.
"On Sunday afternoon, during an exercise by a number of the navy's vessels in Jask and Chabahar waters, the Konarak light support vessel had an accident," the army said on its website.
"The number of this accident's martyrs is 19 and 15 have also been injured," it said. The vessel had been towed to port.
The port of Jask is about 1,270 kilometres south-east of Tehran, on the coast of Iran's Hormozgan province.
Iranian TV said a missile hit a ship that was too close to a target during a firing exercise. Iranian authorities did not name the other ship involved but semi-official media identified it as the frigate Jamaran.
The Iranian military said the incident was being investigated.
A senior military capabilities analyst at Jane's, Reed Foster, said that although the loss of the ship was unlikely to affect Iran's navy significantly, "a replacement will likely take years to come into service".
Overhauled in 2018 and capable of launching missiles, the Dutch-made, 47-metre ship has been in service since 1988 and usually carried a crew of 20. With Iran under stringent US sanctions targeting its military and economy, its navy will be forced to rely on indigenous naval construction to replace lost vessels.
Mr Foster said the accident had damaged the credibility of the Iranian military further, noting that "this is the second high-profile incident in less than half a year where mistakes in missile targeting have resulted in significant loss of life".
Iran mistakenly shot down a Ukrainian airliner shortly after it took off from Tehran on January 8, amid soaring tensions between the Islamic republic and Washington, but only admitted to it days later.
Mr Foster said Tehran has been keen to present the IRGC and the armed forces as defenders of the people "against all threats foreign and domestic", but the Konarak incident is likely to undo some of the work done to restore public trust after the downing of Ukrainian Airlines flight 752.
Iran regularly holds exercises in the area where the friendly fire took place, which is close to the Strait of Hormuz, the narrow mouth of the Arabian Gulf through which 20 per cent of the world’s oil passes.
The accident came amid heightened tensions in the Gulf.
Last month the US accused Iranian fast boats of conducting “dangerous and harassing” manoeuvres near American warships in the northern Arabian Gulf. The Revolutionary Guard denied the accusation, saying the its boats faced "provocative and unprofessional actions" from the US ships while they were conducting drills.
Private security companies claimed the Revolutionary Guard was responsible armed men seizing a Hong Kong-flagged tanker off the Iranian coast near the Strait of Hormuz for a short time. Iran has not acknowledged the incident.