Twenty-three marines were on board the Boeing MV-22B Osprey tilt-rotor aircraft when it crashed on Melville Island, north of Darwin, on Sunday morning during a military exercise for locally based troops.

Northern Territory Chief Minister Natasha Fyles said eight marines were still receiving treatment and that she wanted to "assure their families that they are getting the best care possible and we're working to ensure they're supported".

Authorities have launched an investigation into what caused the crash, and efforts are also under way to recover the remains of the three dead marines.

An exclusion zone has been established around the crash site.

Northern Territory Police Commissioner Michael Murphy told reporters the recovery and investigation would be "prolonged, enduring and complex", adding that officials were expecting to be at the crash site for "at least 10 days".

"Absolutely everything" was being dedicated in terms of resources, he said.

He had earlier told local media that the aircraft hit the ground in "heavy bushland" but emergency responders were helped in their initial efforts by the location of a nearby airstrip.

Being able to land so close to the crash site had allowed casualties to be treated more quickly and "probably saved some lives", he said.

Air traffic control broadcasts from Darwin airport after the incident included communications from the site describing dark smoke and a "significant fire", but Murphy said he could not comment on why the Osprey had crashed.

- Exercise resumes -

"Predators Run" -- the exercise the marines had been participating in -- was paused on Sunday, but an Australian defence official told AFP that it had resumed on Monday morning.

There will, however, be no more exercise activities on the Tiwi Islands, the small group of islands that includes Melville, the scene of the crash.

Northern Australia has become an important staging ground for the US military in recent years, as Washington and Canberra work together to counter China's growing clout in the Asia-Pacific region.

Australian defence minister Richard Marles told public broadcaster ABC on Monday morning that the loss of life would be felt by US and Australian forces.

"These are very tight-knit communities," he said.

US Defense Secretary Lloyd Austin praised the fallen marines, who he said "served our country with courage and pride, and my thoughts and prayers are with their families today, with the other troops who were injured in the crash, and with the entire (US Marine Corps) family".

The Osprey aircraft has a troubled history, blighted by a string of fatal crashes over the years.

Four US marines were killed in Norway last year when their V-22B Osprey aircraft went down during NATO training exercises.

Three marines were killed in 2017 when an Osprey crashed after clipping the back of a transport ship while trying to land at sea off Australia's north coast.

And 19 marines died in 2000 when their Osprey crashed during drills in Arizona.

sm/arb/leg

BOEING