According to Iranian state media, Iran’s largest navy ship sank early Wednesday in the Gulf of Oman after catching fire, the latest blow to the country’s vital infrastructure and military assets in recent months.

The Kharg was deployed to international waters for a training exercise when it caught fire near the port of Jask, according to the semiofficial Tasnim news agency. According to state news agency IRNA, the fire started in the engine room, causing parts of the ship to melt and fall into the sea. Rescue workers attempted to extinguish the fire for 20 hours but were unable to keep it from spreading, according to Tasnim, citing Iran’s navy, which added that all of the nearly 400 crewmembers were safely evacuated from the ship.

Other Iranian media outlets broadcast footage from the Gulf of Oman purportedly showing the burning ship in the distance.

Neither outlet revealed the cause of the fire. The government did not respond immediately to a request for comment. The Kharg was an oil replenishment tanker, capable of carrying large helicopters and designed to allow smaller vessels to embark on extended deployments by supplying them with fuel and dry stores at sea. Its logistics capability also allowed it to transport heavy cargo, such as military equipment, which could raise suspicions in the eyes of its adversaries, according to a 2011 article by U.S. Navy Commander Joshua Himes for the Center for Strategic and International Studies.

The sinking of the ship comes on the heels of a series of recent explosions and fires at key nuclear and military sites, as well as attacks on Iranian naval vessels. While Tehran has not accused anyone of attacking the vessel, the incident comes at a time when tensions between Iran and its regional adversaries are rising. Israel opposes ongoing talks to resurrect the international nuclear deal with Iran and is pressuring the US to do more to contain what it sees as Iran’s malign behavior in the Middle East, including support for militias that pose a threat to Israel.

A new front in the conflict between Iran and Israel has opened up at sea in recent years. Since late 2019, Israel has targeted at least a dozen Iranian vessels bound for Syria, according to US and regional officials, with the majority of them carrying oil, which Israel claims funds Iranian-backed extremist groups. The Israeli military has refused to comment on the alleged Iranian ship attacks. A fire in Iran’s southern port of Bandar Abbas in July damaged seven ships and raised the possibility of coordinated sabotage.

Iran has also been hit by devastating natural disasters. Last May, an Iranian warship collided with one of the country’s naval vessels during a training exercise near a strategic Persian Gulf waterway, killing at least 19 sailors.

Iran has also accused Israel of being behind several recent attacks on its nuclear program, including two fires and explosions at its Natanz nuclear facility in April and July of this year, as well as the November assassination of its top nuclear scientist. Israel usually doesn’t comment on such allegations.

The sunk vessel, a modified OL-class vessel originally built for the Shah by the United Kingdom in the late 1970s, was part of an aging Iranian fleet that has deteriorated over the years due to US and international sanctions. It was delivered to Iran in 1984 and has been in service since then.