The MI-171 aircraft, which was on a training mission, came down at a village in the Thach That district.

Lieutenant General Vo Van Tuan, deputy chief of the general staff, told the BBC that the helicopter had crashed due to technical failure.

The dead and injured were all military personnel, he added.

"Our priority now is to look after the five injured, some who are in serious condition," he said.

Eyewitnesses said the helicopter appeared to run into difficulties before it crashed and caught fire.

"The aircraft was flying low, it hit the ground and exploded at the same time," a local resident told AFP news agency.

"It seemed like the pilot wanted to divert the plane from hitting the local market and houses, so it ran into a field."

Most of the Vietnamese military's ageing fleet has been purchased from Russia.

A MIC-21 jet fighter crashed on the same day four years ago but the pilots managed to parachute to safety.

Another accident in 2008 saw five military pilots killed when their aircraft crashed on the outskirts of Hanoi.