Klyuchevskaya, one of the world's highest active volcanoes, rising to a height of 4,750 meters (15,584 feet), is emitting lava at an altitude of 4,200-4,300 meters (about 14,000 feet), the service said.
"The authorities should warn people about a possible danger and take proper security measures," said Alexei Ozerov, a senior seismology researcher, adding that the lava flow would grow more powerful.
Streams of lava with a temperature of about 1,100 C( (2,012 F() are encountering ice caps, producing powerful explosions of vapor. Experts are warning of devastating mudflows that can reach 500 meters (1,640 feet) in width and can descend the volcano's eastern slopes.
Local officials said "the population and companies working in the area that could be affected by mudflows" had been told to take precautions.
The current lava eruption on Klyuchevskaya began February 15, and the previous emission occurred in January-May 2005, when the volcano caved in by about 50 meters (150 feet) and the village of Klyuchi, located about 40 kilometers (25 miles) from the volcano, was covered in ash.
The largest gas and ash emission in five years from Mount Shiveluch, Kamchatka's northernmost volcano with an altitude of 3,283 meters (10,771 feet), began at 3 p.m. local time (6 a.m. Moscow time, 2 a.m. GMT) at an altitude of 6 kilometers (3.7 miles) or 9 kilometers (5.6 miles) above sea level.
Clouds of volcanic ash are spreading to the northeast of the mountain, posing a threat to people's health and air flights. Volcanic tremors are being registered in the nearby area.
There are more than 150 volcanoes on Kamchatka, 29 of them active. Experts say volcanoes' activity has recently increased on the Peninsula.