An Iranian missile fired during a training exercise in the Gulf of Oman struck a support vessel near its target, killing 19 sailors and wounding 15, Iran's state media reported on Monday, amid heightened tensions between Tehran and the US.

The statement significantly raised the death toll in Sunday's incident from what was reported just hours earlier, when Iran's state media said at least one sailor was killed.

The Konarak, a Hendijan-class support ship, which was taking part in the exercise, was too close to a target during an exercise on Sunday when the incident happened, the reports said. The vessel had been putting targets out for other ships to target. The media said the missile struck the vessel accidentally.

The friendly fire incident happened near the port of Jask, some 1,270ms southeast of Tehran, in the Gulf of Oman, state TV said.

A local hospital admitted 12 sailors and treated another three with slight wounds, the state-run IRNA news agency reported.

Iranian media said the Konarak had been overhauled in 2018 and was able to launch sea and anti-ship missiles. The Dutch-made, 47-meter vessel was in service since 1988 and had capacity of 40 tons. It usually carries a crew of 20 sailors.

Iran regularly holds exercises in the region, which is closed to the strategic Strait of Hormuz, the narrow mouth of the Persian Gulf through which 20% of the world's oil passes. The US Navy's 5th Fleet, which monitors the region, did not immediately respond to a request for comment.

Iranian media rarely report on mishaps during its exercises, signaling the severity of the incident. This incident also comes amid months of heightened tensions between Iran and the US since American President Donald Trump unilaterally withdrew from Tehran's nuclear deal with world powers in 2018 and imposed crushing sanctions on the country.