More than 20,000 firemen are battling lighting-triggered forest fires in northeast China as continuous hot weather undermines their efforts, forest fire prevention authorities said.

The fire, spotted Saturday, continued to spread Wednesday due to high temperatures after having weakened overnight, said Sun Zhagen, deputy director of China's National Forest Fire Prevention Headquarters.

The forest fire first occurred in a part of the Greater Hinggan Mountains in Inner Mongolia Autonomous Region and then extended to the neighboring Heilongjiang Province.

Rescuers were still struggling to put out the fire at more than 30 sites in the two regions, said a spokesman with the fire fighting headquarters at Huzhong, a major fire site.

Further, another 6,000 fire fighters and additional equipment would be sent to control the fire in Heilongjiang, said the spokesman.

"As it is too hot to get close to the scene to put out the fire, we have to build fire barriers during the day and battle the blaze at night," said Pang Zhiqiang, a forest policeman.

Sparks are being buried under the burnt debris of wood, often as deep as 30 cm and easily set on fire again by wind, said rescuers at Huzhong District.

"We felt thirsty, dizzy and had trouble breathing after staying in the burnt forest for just 10 minutes", one fireman said.

Each of the rescuers, carrying about 25 kg of equipment on his back, could only carry four bottles of drinking water, a rescuer with the Heilongjiang forestry police force told a Xinhua reporter. "If the bottle water is not enough, we drink the river water, though there are worms in it."

Some thirsty fire fighters said they even dug into the ground and breathed in the moisture. Also, many rescuers have become exhausted after remaining awake and working for four days.

Temperatures in the Greater Hinggan Mountains Region have been hovering over 37 degrees Celsius recently and seven counties and districts have witnessed record-high temperatures, said Na Jihai, chief of the Heilongjiang Provincial Meteorological Bureau.

Temperatures in Huzhong District hit 39.7 degrees Celsius Saturday, Na said.

Fire-control experts in Inner Mongolia also said such an extremely-hot weather was rarely seen over the past six decades.

The two regions have dispatched three aircraft and 10 cloud seeding rockets to conduct artificial precipitation operations, according to local meteorological departments.

The dry and hot weather will continue for the next three days, but a light rain is forecast for Thursday, according to the National Meteorological Center of China Meteorological Administration.

"This is a battle between human beings and nature. We have to work hard to win," Sun Zhagen said.