Three US Marines died during military exercises in northern Australia on Sunday, the US military said, in a helicopter crash that Prime Minister Anthony Albanese called "tragic".
Five others were "transported to Royal Darwin Hospital in serious condition", Marine Rotation Force - Darwin said in a press release.
It said they were among 23 Marines on a MV-22B Osprey helicopter during routine training exercises and that the cause of the crash was being investigated.
The crash occurred on the remote Tiwi Islands after 9am (0100 GMT), said Northern Territory Police Commissioner Michael Murphy.
Australian personnel were not involved in the crash that occurred during Exercise Predators Run 2023, Albanese said.
"Our focus as a government and as a department of defence is very much on incident response and on making sure that every support and assistance is given at this difficult time," the prime minister told a previously scheduled press conference in Western Australia.
About 2,500 personnel from Australia, the US, the Philippines, Indonesia and East Timor were taking part in the exercises, Sky News reported.
The US and Australia, a key ally in the Pacific, have been stepping up military cooperation in recent years in the face of an increasingly assertive China.
Four Australian soldiers were killed last month during large bilateral exercises when their helicopter crashed into the ocean off the coast of Queensland.