Eight US marines were in hospital on Monday -- one of them in intensive care -- following a military aircraft crash in northern Australia that killed three of their comrades, authorities said.

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 underway to recover the remains of the three dead marines.

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