Two people were killed when a US military attack helicopter crashed on a mountain road in South Korea.

A US military spokesman said the helicopter, believed to be an AH-64 Apache, crashed in the city of Wonju, 70 kilometres southeast of Seoul.

'The helicopter was burnt to ashes after catching fire,' a firefighter in Wonju said, adding two people aboard died at the scene, AFP reported.

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Search: Firefighters wade through debris of a crashed US helicopter, believed to be an AH-64 Apache, on a mountain road in Wonju, about 70 kilometres southeast of Seoul

Wreckage: The US military attack helicopter is believed to have hit a high voltage line. Investigators are still looking into why the chopper came down in South Korea's Gangwon province

Police and firefighters say they recovered the bodies of the two, which have not yet been identified, from the wreckage.

The AH-64 Apache helicopter that belongs to the 2nd Infantry Division of the U.S. troops in Korea crashed at about 6:22 p.m. after presumably taking off from Osan Air Base in Pyeongtaek, south of Seoul.

'I went out after hearing a 'bang' sound twice and saw flames rising into the sky from the side of the road,' Yonhap news agency reported a resident as saying.

Chopper: A stock image of a US AH-64 Apache helicopter

The authorities suspect the attack helicopter hit a high-voltage line or a steel tower seen from the facts that the line was among the wreckage and the upper part of the tower was damaged.

The chopper went down onto the local road some 500 meters away from a village and was burnt to ashes after catching fire.

The firefighter said the exact cause of the crash was unknown although no additional property damage or human casualties were reported.