TEHRAN, Iran (AP) — The largest warship in the Iranian navy caught fire and later sank Wednesday in the Gulf of Oman under unclear circumstances, the latest calamity to strike one of the country’s vessels in recent years amid tensions with the West.

The blaze began around 2:25 a.m. and firefighters tried to contain it, the Fars news agency reported, but their efforts failed to save the 207-meter (679-foot) Kharg, which was used to resupply other ships in the fleet at sea and conduct training exercises. State media reported 400 troops on board fled the vessel, with some 20 suffering injuries.

The vessel sank near the Iranian port of Jask, some 1,270 kilometers (790 miles) southeast of Tehran on the Gulf of Oman near the Strait of Hormuz — the narrow mouth of the Persian Gulf. Satellite photos from Planet Labs Inc. analyzed by The Associated Press showed the Kharg off Jask with no sign of a fire as late as 11 a.m. Tuesday.

Photos circulated on Iranian social media of sailors wearing life jackets evacuating the vessel as a fire burned behind them. Fars published video of thick, black smoke rising from the ship early Wednesday morning. Satellites from the U.S. National Oceanic and Atmospheric Administration that track fires from space detected a blaze near Jask that started just before the time of the fire reported by Fars.

Iranian officials offered no cause for the fire aboard the Kharg, though they said an investigation had begun. It comes after a series of mysterious explosions that began in 2019 targeting commercial ships in the Gulf of Oman. The U.S. Navy later accused Iran of targeting the ships with limpet mines, timed explosives typically attached by divers to a vessel’s hull.

Iran denied that, though U.S. Navy footage showed Revolutionary Guard members removing one unexploded limpet mine from a ship. The attacks came amid heightened tensions between the U.S. and Iran after then-President Donald Trump unilaterally withdrew America from Tehran’s nuclear deal with world powers. Negotiations on saving the accord continue in Vienna.

In April, an Iranian ship called the MV Saviz believed to be a Guard base and anchored for years in the Red Sea off Yemen was targeted in an attack suspected to have been carried out by Israel. It escalated a yearslong shadow war in the Mideast between the two countries, ranging from strikes in Syria, assaults on ships and attacks on Iran’s nuclear program.

The Israeli prime minister’s office did not respond to a request for comment Wednesday regarding the Kharg.

State TV and semiofficial news agencies on Wednesday referred to the Kharg, named after the island that serves as the main oil terminal for Iran, as a “training ship.” It was one of a few vessels in the Iranian navy capable of providing replenishment at sea for other ships. It also can lift heavy cargo and serve as a launch point for helicopters.

Like much of Iran’s major military hardware, the Kharg dated back to before Iran’s 1979 Islamic Revolution. The warship, built in Britain and launched in 1977, entered the Iranian navy in 1984 after lengthy negotiations. That aging military equipment has seen fatal accidents as recently as Tuesday, when a malfunction in the ejector seats of an Iranian F-5 dating back to before the revolution killed two pilots while the aircraft was parked in a hangar.

In recent months, the navy launched a slightly larger commercial tanker called the Makran that it converted into serving a similar function as the Kharg.

The sinking of the Kharg marks the latest naval disaster for Iran. In 2020, during an Iranian military training exercise, a missile mistakenly struck a naval vessel near Jask, killing 19 sailors and wounding 15. Also in 2018, an Iranian navy destroyer sank in the Caspian Sea.

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Gambrell reported from Dubai, United Arab Emirates. Associated Press writer Joseph Krauss in Jerusalem contributed to this report.