A second big fire broke out in China's largest forest in the Greater Hinggan Mountains on the same day a cross-border fire from Russia was extinguished.

The fire broke out at 12:15 pm on Tuesday in the Beidahe forest in the Inner Mongolia autonomous region. More than 8,300 firefighters are working to extinguish the blaze, according to regional fire authorities.

By press time, the fire had engulfed about 50 square kilometers of forest. The firefighters - 2,590 of whom are armed police officers - and 12 helicopters have been sent.

Strong winds, mountainous terrain, and dry vegetation made the firefighting "extremely difficult", according to an official statement from the fire authorities.

The blaze was thought to be caused by a stoker employed at a forestry station who improperly disposed of ashes and other residue from a fire. The suspect is in custody, the State Forest Fire Prevention Headquarters said.

Wang Haizhong, deputy director of the general office of the headquarters, warned that over the next 10 days China will face an increasing risk of forest fires due to an expected rise in temperatures across the nation.

Wang was quoted in a Xinhua interview as saying that China still lags behind developed countries in terms of investment in forest fire prevention.

In some poorly prepared western provinces, he said, the expenditure on fire prevention was less than 10 cents per hectare of forest, while in the United States and Canada it can reach $2 per hectare.

The temperature in Inner Mongolia this spring is higher than in previous years, posing a higher fire risk for the region's forests.

A cross-border fire broke out Sunday in another area. That fire was put out on Tuesday and cleanup is underway.

Xinhua contributed to this story.