The 4,750 m Klyuchevskoy volcano is spewing out rocks, ash and gases, said Alexei Ozerov, a scientist with the Institute of Volcanology and Seismology of the Far Eastern branch of the Russian Academy of Sciences.
"The luminescence in the volcano's crater is becoming more intensive, which testifies to the appearance of new lava," Ozerov said, adding that the eruption would probably last from two or three weeks to six months.
Continuous flows of lava running down Klyuchevskoy's slopes could trigger mudslides as the molten rock melts the snow and ice, endangering people living in nearby villages, the scientist said.
The volcano eruption is also a threat to aircraft flying overhead as volcanic ash could cause planes considerable damage, he added.
The last powerful eruption of the Klyuchevskoy volcano took place in 2005.