One cargo helicopter of the United States Marine Corps took an emergency landing in northern Thailand during an annual Cobra Gold exercise on Wednesday, leaving four marines injured.

Four of the seven US marines aboard the CH-46 Sea Knight helicopter were injured in the accident, according to the latest report from local MCOT TV.

Two of them suffer serious burnt wounds.

Thai Air Force Chief ACM Prajin Jantong said the US Marine Corps helicopter landed on a craggy terrain in Puhin Rongka mountain range in Pitsanuloke province.

The cause of the accident remains unknown. Two helicopters, one from the United States and one from the Thai army, were dispatched to the area, said Jantong.

The US Marine Corps has joined the US Army, US Navy and U. S. Air Force in the 11-day multinational military exercise in Thailand which is scheduled to end on Friday. An estimated 13,000 servicemen from seven countries were participating in the Cobra Gold exercise, including those from Singapore, Malaysia, Indonesia, Japan, South Korea, the United States and Thailand.

In addition to Pitsanulok, the drill has also been conducted in the northern provinces of Sukhothai, the eastern seaboard provinces of Chonburi and Chantaburi and the central province of Lopburi. Varied training programs included computer-aided, command- post operations, staff and commanding officers engagements, field training operations and humanitarian and civic assistance programs were conducted.

Besides the CH-46 Sea Knight and other cargo helicopters, a number of jet fighters, bombers and attack helicopters from all participating countries have joined the US-led exercise, considered to be of the largest scale in the Asia-Pacific region.

Military officers from several countries viewed the 32nd annual exercise as observers including those from China, Russia, the United Arab Emirates, Brunei and Laos.