One of Iran’s largest navy ships has sunk after catching fire near the Strait of Hormuz under unclear circumstances.

The Kharg, named after an island nearby that serves as a key oil terminal, caught fire in the early hours of Wednesday local time and more than 20 hours of rescue operations failed to save it, the army said.

An image taken late at night showed crew in life jackets running away with a fire raging behind them. Another image, taken in daytime, showed heavy smoke billowing to the sky and fire still burning.

Iran’s army also identified the Kharg as a “training ship” and confirmed there were almost 400 crew and trainees on board, before adding all of them were safely evacuated.

A local army official told the semi-official Tasnim news agency that 20 people suffered minor burns.

No reason was provided for the cause of the fire but an army official told Iran’s IRNA the fire started in the engine room and parts of the ship’s body melted and fell into the sea.

The vessel sank close to the port of Jask in the south of the country, near the Strait of Hormuz, a strategic waterway.

The vessel was built in Britain and launched shortly before Iran’s 1979 Revolution and entered the Iranian navy in 1984 after several years of negotiations.

The army official told Tasnim that much of its equipment had been overhauled by Iranian engineers in recent years.

It was capable of providing replenishment at sea to help other ships, could lift heavy cargo and also had launch and landing pads for helicopters.

The Kharg’s sinking marks the latest big naval incident for Iran in recent years.

Last year, a missile was mistakenly fired during a military training exercise at sea, killing 19 sailors and wounding 15 others.

An Iranian navy destroyer also sank in the Caspian Sea in 2018.