HOHHOT - A four-km firing line extended from Russia to primitive forest to the north of Greater Hinggan Mountain in North China's Inner Mongolia autonomous region by Thursday afternoon, said local forestry police.

More than 450 firefighters have reached the scene in the Yimuhe Forest Farm run by Wuma Forestry Bureau by helicopter.

It is difficult to contain the blaze as the fireground is so inaccessible, with poor communications and too much combustible undergrowth, said the region's forestry bureau.

The State Forest Fire Prevention Headquarters has activated the top-level red alert.

An emergency work team with the North China Aeronautic Center for Forest Protection under the State Forestry Administration is on their way to the scene. Eight helicopters have been despatched.

Another 300-strong firefighters are on the way by helicopter.

The blaze broke out at 12:30 am Wednesday.