US jets drop unarmed bombs on Great Barrier Reef

US fighter jets dropped four unarmed bombs on Australia's Great Barrier Reef in an "emergency jettison" during a training exercise, officials said on Sunday, ruling out any risk to the public or environment.

The US 7th Fleet said two Harrier aircraft dropped the ordnance on the iconic reef's marine park off the Queensland coast on Tuesday.

"The selected emergency jettison area was in a deep channel away from the reef to minimize the possibility of reef damage," the US 7th Fleet said in a statement.

"It is approximately 50 to 60 metres deep and does not pose a hazard to shipping or navigation."

An Australian Defence Force spokesman told Australian Associated Press that the bombs posed "minimal risk or threat to the public, the marine environment or civilian shipping transiting the reef area".

"Defence is working closely with the US military, the Australian government and environmental organisations to ensure there is no danger... now or in the future," he said.

US and Australian officials are seeking to issue an appropriate navigation notice until charts can be updated showing the location of the unexploded ordnance, the 7th Fleet said.

The two AV-8B Harrier planes had intended to drop the bombs on a range on a nearby island but were unsuccessful despite several attempts.

"After being unable to expend their ordnance during a planning training mission, and having insufficient fuel to reach their pre-designated jettison area, the pilots jettisoned the ordnance in an unarmed condition on July 16," the 7th fleet said.

The fleet said each Harrier jettisoned one BDU 45, which are inert ordnance, and one GBU 12, which were dropped in an unarmed state.

About 28,000 Australian and US military are involved in training exercise Talisman Saber in Queensland, which began on July 15 and ends on August 5.