A search is under way for three missing US Marines after a military heli-plane crashed off the coast of Queensland on Saturday.

The aircraft, an MV-22 Osprey, crashed near the Shoalwater Bay military training area, north of Rockhampton, at about 4pm.

An MV-22 Osprey pictured at an RAAF Base in Townsville. Credit:US Army

A US defence official said there were 26 people on board the aircraft when it crashed, and only twenty-three of those had been rescued by Saturday night.

Other US military aircraft and boats responded to the area, where they are combing the water for any sign of the missing service members.