US admits fighter jets dropped unarmed bombs on the World Heritage-listed Great Barrier Reef

US and Australian authorities have embarked on an investigation

US fighter jets dropped unarmed bombs on the World Heritage-listed Great Barrier Reef last week after a training exercise went wrong, it emerged today, in a revelation that outraged environmentalists.

Two planes were given permission to dump four weapons on the marine park after a mission to drop them on a bombing range on a nearby island had to be aborted.

The incident took place during joint US-Australian exercises over central Queensland last Tuesday.

The US 7th Fleet said in a statement that the bombs were jettisoned in more than 50 metres of water, "in a deep channel away from the reef, to minimise the possibility of reef damage".

But conservationists were furious, with Larissa Waters, a Queensland senator with the Australian Greens party, calling the incident "outrageous". "I mean, have we gone completely mad?" she asked. "Is this how we look after our World Heritage area now? Letting a foreign power drop bombs on it?"

The two AV-8B Harrier jets from the 31st Marine Expeditionary Unit were supposed to drop the bombs on the Townshend Island range, but air traffic controllers reported "hazards" in the area, and after several unsuccessful attempts the pilots aborted the mission.

The jets - which were launched from the aircraft carrier USS Bonhomme Richard in the Pacific Ocean - jettisoned the weapons because they were low on fuel and could not land with their bomb loads, according to the US Navy statement.

It said there was no danger to shipping or navigation.

The incident happened on the second day of the biennial joint military exercise, named Talisman Sabre, which  involves about 28,000 personnel over three weeks. The bombs each weighed 226 kilos, according to the US TV network NBC.

US and Australian authorities have embarked on an investigation.

The Barrier Reef - the world's largest coral reef, stretching 2,600 kilometres along Australia's east coast and rich in marine life - is already vulnerable thanks to warmer seas, industrial port development and pollution run-off from farms.

The UN body UNESCO has warned Australia that the reef will be listed as threatened unless steps are taken to protect it.