The aircraft, an MV-22 Osprey, is believed to have crashed on Saturday afternoon, with several of those on board unaccounted for.

Other military aircraft and ships have been sent to the area, where they are combing the water for any sign of the missing service members.

In a statement, the US Marine Corps Public Affairs Office said there had been a "mishap".

"The United States Marine Corps confirms there is an active search and rescue mission ongoing for service members involved in an MV-22 mishap off the east coast of Australia," the statement said.

"Ships, small boats and aircraft from the 31st Marine Expeditionary Unit and Bonhomme Richard Expeditionary Strike Group are conducting the search and rescue operations.

"We will provide more details as they become available."

US forces were in Australia for Talisman Saber, a joint military exercise involving 33,000 US and Australian troops which is held each two years.

The combat readiness training, which was expected to end in late July, was based at Shoalwater Bay Training Area, near Rockhampton.

MV-22s have been involved in several serious crashes, some of them fatal, in the 10 years they have been used by the US military.

An MV-22 had a "hard landing" during a raid in Yemen in January this year, injuring three soldiers on board. The $75 million aircraft was then destroyed by an air strike.

In December 2016, the US grounded its fleet of the aircraft in Japan, after one crash-landed off Okinawa. That incident was also described as a "mishap".