There was no immediate word on casualties, but the military denied reports the aircraft had been carrying radioactive material when it came down near Glamis, just 20 miles (35 kilometers) from the Mexican border.

"We can confirm that an aircraft belonging to 3d Marine Aircraft Wing crashed near Glamis," a spokesman told AFP.

"Five Marines were onboard the aircraft, and we are awaiting confirmation on the status of all members of the crew.

"Military and civilian first responders are on site. Contrary to social media rumors, there was no nuclear material on board the aircraft."

The aircraft was identified as an MV-22B Osprey based at Marine Corps Air Station Camp Pendleton.

The Osprey is a so-called "vertical takeoff and landing" aircraft that has rotary wings which can be directed upward to give it the maneuverability of a helicopter, or forward to give it the range of a plane.

The US military has suffered a number of crashes with the aircraft, including an accident in Norway in March that left four Marines dead.