Most of the fires were blazing in the Amur and Khabarovsk regions, the regional forestry protection center reported.

Some 4,000 firefighters, backed by planes, helicopters and special equipment, have been mobilised to fight the fires, which are raging out of control in an area that has recently seen little rainfall.

Investigators said they were examining the possibility that the fires were started deliberately, the ITAR-TASS news agency reported, quoting officials in Moscow.

A coordinated arson campaign could have been launched for commercial reasons, since fire-damaged wood costs less to export, officials told a press conference.

"The human factor is the cause of these fires in 90 percent of cases," Valentin Simuchenkov, deputy state prosecutor for the Far East region, told reporters.

Such blazes destroy a million hectares of vegetation in Far East Russia each year.

SPACE.WIRE