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Three US marines have been killed and 20 injured after a military helicopter crashed during exercises in Australia.

The MV-22B Osprey came down on its way to the remote Tiwi islands north of Darwin. Five of the marines are said to be in a serious condition.

They were taking part in Predators Run, involving 2,500 troops from the US, Australia, the Philippines, East Timor and Indonesia.

Only US personnel were on board the aircraft.

The incident took place on Melville Island north of the Northern Territory capital Darwin on Sunday.

Australian Prime Minister Anthony Albanese described the incident as tragic and said every assistance would be given to the injured.

"Five marines have been returned to Darwin for treatment with the remaining being triaged at the scene," said local police commissioner Michael Murphy.

"Additional police and defence personnel have been deployed to Melville Island to support operations, triage injured people and return them to Darwin and also maintain the crash scene while investigations continue."

The incident is the second fatal helicopter crash during joint exercises in northern Australia in less than a month.

On 29 July four Australian soldiers were presumed dead after their 45 MRH-90 Taipan crashed off the coast of Queensland while participating in Exercise Talisman Sabre, the largest bilateral military training exercise between Australia and the US.

The Osprey is a hybrid combining features of helicopters and turbo-prop planes which can travel much faster than a conventional helicopter.

But it has a troubled history, with a fatal crash in Norway last year killing four marines. Three others died in 2017 when their aircraft clipped the back of a transport ship off northern Australia.