Image caption The Great Barrier Reef is the world's largest coral structure rich in marine life

US fighter jets dropped four bombs on the Great Barrier Reef off Australia's coast during a training exercise that went wrong, it has emerged.

The two planes jettisoned four bombs - two inert and two unarmed - in more than 50m (165 ft) of water, away from coral, to minimise damage to the World Heritage Site, the US navy said.

The jets were to bomb an island range nearby, but the mission was aborted.

The AV-8B Harriers were low on fuel and could not land loaded, the navy added.

The incident happened during the training exercise Talisman Saber, involving US and Australian military personnel.

The two jets had been instructed to target the bombing range on Townshend Island.

However, the mission was aborted when hazards were reported in the area.

"It was not safe to drop the bombs. There were civilian boats right below them," Commander William Marks of the US Seventh Fleet told ABC radio.

The planes then dropped the bombs in the marine park off the coast of Queensland. None of the devices exploded.

The bombs - each weighing 225kg (500lbs) - do not pose a hazard to shipping, the US Navy says, and charts are to be updated to show the unexploded ordnance.

"There is minimal environmental impact," said Cmdr Marks. "It is a safe situation for the environment, for shipping, for navigation."

However, the US Navy said it was open to the idea of recovering the bombs from the seabed.

"If the [Great Barrier Reef] park service and the government agencies of Australia determine that they want those recovered, then we will coordinate with them on that recovery process," said Seventh Fleet spokesman Lt David Levy.

The Great Barrier Reef is the world's largest coral structure rich in marine life.

It stretches for more than 2,600km (1,680 miles) along Australia's eastern coast.