Three U.S. Marines died in an aircraft crash off the coast of northern Australia on Sunday while transporting troops during a routine military exercise, officials said.

Five others were "transported to Royal Darwin Hospital in serious condition", Marine Rotation Force - Darwin said in a press release.

They were among 23 Marines on the MV-22B Osprey tilt-rotor aircraft when it crashed, the statement said, adding the cause of the crash was being investigated.

The crash occurred on the remote Tiwi Islands at approximately 9.30 a.m. (0130 GMT), the statement said.

Australian Prime Minister Anthony Albanese called the incident "tragic", saying the Osprey was supporting the Exercise Predators Run 2023 military exercise. No Australian personnel were not involved in the crash, he 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 U.S., the Philippines, Indonesia and East Timor were taking part in the exercises.

The U.S. 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.

"Australian and US personnel have stood shoulder to shoulder for more than a century," Albanese and the Defence Minister Richard Marles said in a statement. (Reuters)