US helicopter
US helicopterReuters

A number of passengers on board a United States Marine helicopter were seriously injured after the aircraft was forced to make a "hard landing" Wednesday during military exercises in northern Thailand, the U.S. Embassy in Bangkok said.

Embassy spokesman Walter Braunohler said there were no fatalities in the incident, which involved a CH-46 Sea Knight helicopter carrying U.S. personnel taking part in the Cobra Gold joint military drills.

Two of the passengers were in need of emergency surgery, one sustained more minor injuries, while another three escaped unscathed, a source with knowledge of the incident said, according to AFP.

Cobra Gold is the largest US multilateral exercise in the Asia-Pacific, bringing together thousands of troops from the U.S., Thailand and other countries for field training.

The annual drills are a key part of the U.S. military relationship with Thailand, whose air bases and ports remain vital to the American military's logistical network in Asia.