Three United States Marines are dead, and five others are in serious condition after an aircraft crashed during a military exercise in Australia on Sunday, officials say.

In total, 23 Marines were on board the MV-22B Osprey aircraft during the incident, which took place during a "routine training exercise" on Melville Island in Australia at 9:30 a.m. local time, according to a statement from the Marine Rotational Force of Darwin. The statement noted that the situation was still developing and that recovery efforts were "ongoing."

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The five injured Marines were transferred to Royal Darwin Hospital for medical assistance, according to the statement. Northern Territory Chief Minister Natasha Fyles said earlier Sunday that other Marines were being treated at the scene.

“We are working incredibly hard and as fast as we can to make sure we can get people to treatment,” Fyles said.

Australian Prime Minister Anthony Albanese said that initial reports suggest that only Americans, and no Australians, were on board the aircraft.

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“Our focus as a government and as the Department of Defense is very much on incident response and on making sure that every support and assistance is given at this difficult time,” Albanese said.

The cause of the incident is now under investigation, though the statement said that it occurred as part of “Exercise Predators Run,” which is described as “a multilateral combined arms exercise” held in the Northern Territory. It involves personnel from the Australian Army, Malaysian Armed Forces, the Armed Forces of the Philippines, and the U.S. Marine Corps.