The Iranian navy’s largest warship sank Wednesday in the Gulf of Oman after catching fire.

The blaze broke out on the 679-foot Kharg in Iranian waters, and it sank off the port of Jask, close to the Strait of Hormuz, after firefighting efforts that lasted nearly a day, Iran’s semiofficial Fars News Agency reported. The Associated Press reported that the fire began at 2:25 a.m., while Fars News said it began at 11 a.m. the day before.

The vessel, built by Britain in 1977 and part of Iran’s maritime fleet since 1984, had been used for training exercises and to resupply ships at sea, according to Iranian state media.

Around 400 sailors were aboard the 33,000-ton ship at the time of the fire, and about 20 were injured, state media reported.

The incident came at a time of heightened tensions between Iran and Israel, along with its Western allies, and as the United States and Iran negotiate over nuclear activity and sanctions.

A spokesperson for Iran’s navy told Fars News that the cause of the incident is under investigation. Iranian authorities have offered no official explanation.

“A 20-hour effort to extinguish the fire by the ship’s damage-control team, which were joined by firefighting crews as well as military and civilian forces from other nearby vessels, failed to save Kharg,” the spokesperson said.

According to the official Islamic Republic News Agency, the ship returned to use three years ago after five years of repairs, which included improvements to its boiler system. The news agency cited a navy officer as saying the fire began in the engine room.

The loss of the ship was not the only major incident Iran faced Wednesday: Another large fire broke out at an oil refinery that serves Tehran, Iran’s capital. It was not immediately clear what the cause was or whether there were casualties.

Much of Iran’s major military equipment was acquired before its 1979 Islamic revolution, after which Western arms embargoes made it difficult for the country to upgrade and repair its arsenal. Fatal military accidents there are not uncommon. On Tuesday, an aging Iranian F-5 fighter aircraft malfunctioned while parked in a hangar, killing two pilots, who were mistakenly ejected, the AP reported.

In 2020, a missile mistakenly struck a vessel near Jask during an Iranian military drill, killing 19 troops.

Iran’s military recently introduced a commercial vessel, the Makran, which is slightly larger than the Kharg, to serve some of the functions of the now-sunken ship.

Since 2019, a number of explosions have been reported on ships in the Gulf of Oman, with Israel and Iran exchanging blame for the apparent attacks as part of the shadow war unfolding regionwide between the two countries.