The fleet replenishment tanker Kharg had caught fire on Tuesday near the port of Jask on the Gulf of Oman.

The fire broke out in "one of the systems" of the vessel, a navy statement said without elaborating.

Firefighting efforts continued "for 20 hours" before the ship went down, it said.

"Considering the spread of the fire, the mission to save the Kharg failed and it sank in waters off Jask."

It added that the vessel had left for a "training mission" in international waters days ago.

Last year, an Iranian warship was hit by friendly fire during a naval exercise off Jask, killing the 19 sailors onboard.

The port lies close to the Strait of Hormuz, the strategic chokepoint at the head of the Gulf through which a fifth of world oil output passes.

Jask is also important to Iran's economy as the site where the government plans to build the country's second-largest oil export terminal.

A 1,000-kilometre (620-mile) pipeline from Bushehr province on the Gulf to Jask was put into service just days ago, the government said.

It will provide a new bypass route for oil exports avoiding the Strait of Hormuz.

In recent months, there have also been reported attacks on Iranian shipping linked to its arch foe Israel.

In April, Iran said its freighter Saviz was hit by an "explosion" in the Red Sea, after media reports said Israel had struck the ship.