Two US fighter jets have dropped four unarmed bombs on Australia's Great Barrier Reef after a training exercise went wrong.

The two jets dropped an inert bomb and an unarmed explosive bomb in the marine park off the coast of Queensland state, the US 7th Fleet said in a statement yesterday.

A total of four bombs were dropped in more than 50 metres of water away from coral to minimise possible damage to the reef.

None of the bombs, dropped from AV-8B Harrier jets, exploded.

The jets dropped the bombs because they were low on fuel and could not land with their bomb load.