Firefighters board a helicopter to extinguish fire in a national nature reserve in the Greater Hinggan Mountains, North China's Inner Mongolia autonomous region, June 3, 2018. Nearly 4,000 police and fire fighters are battling forest fires in North China's Inner Mongolia autonomous region, local fire authorities said Sunday. [Photo/Xinhua]

Firefighters work to extinguish the blaze at a forest in the Greater Hinggan Mountains, North China's Inner Mongolia autonomous region, June 3, 2018. [Photo/Xinhua]

Firefighters work to extinguish the blaze at a forest in the Greater Hinggan Mountains, North China's Inner Mongolia autonomous region, June 3, 2018. [Photo/Xinhua]

Firefighters work to extinguish the blaze at a forest in the Greater Hinggan Mountains, North China's Inner Mongolia autonomous region, June 3, 2018. [Photo/VCG]

Firefighters work to extinguish the blaze at a forest in the Greater Hinggan Mountains, North China's Inner Mongolia autonomous region, June 3, 2018. [Photo/VCG]

Firefighters work to extinguish the blaze at a forest in the Greater Hinggan Mountains, North China's Inner Mongolia autonomous region, June 3, 2018. [Photo/VCG]

Firefighters work to extinguish the blaze at a forest in the Greater Hinggan Mountains, North China's Inner Mongolia autonomous region, June 3, 2018. [Photo/VCG]