SYDNEY - The US Navy said on Monday it was forced to drop four unarmed bombs on to Australia's fragile Great Barrier Reef during a military exercise last week but the bombs did not explode and did not pose a hazard to shipping or navigation.

Two Harrier fighter jets dropped four bombs off the coast of tropical Queensland state on Tuesday, according to the US Navy, which is taking part in a biennial joint exercise with the Australian Defence force.

The pilots of the jets had intended to drop the four bombs, two inert and two unarmed, on the Townshend Island bomb range, but were told there were hazards in the area, Commander William Marks of the US Seventh Fleet told Australian Broadcasting Corporation radio on Monday.

"It was not safe to drop the bombs. There were civilian boats right below them," said Marks. The US Navy has also said in a statement the jets were low on fuel.

"The Harriers ... needed to get back to the ship, and so they conducted an emergency jettison," Marks said.

The bombs were dropped about 16 nautical miles south of Bell Cay in the Great Barrier Reef Marine Park.  

The World Heritage-listed Great Barrier Reef is a popular tourist site worth about 6 billion Australian dollars ($6.1 billion) a year to the Australian economy.

The United Nations warned in June 2012 that the world's largest living structure faced threats including climate change, rising seaborne traffic and an invasive starfish plague.

The bombs are lying in a deep channel away from the reef to "minimize the possibility of reef damage", the US navy said. The site was approximately 50 to 60 metres (165-200 feet) deep and did not pose a hazard to shipping or navigation.

"We are coordinating with Australian officials to ensure an appropriate navigation notice is issued until charts can be updated showing the location of the unexploded ordnance," the US Navy statement said.

Australian media reported the US Navy was planning to recover the four bombs, although the Navy was not immediately available to comment on the reports.