Russian geologists said that a "strong explosive eruption" of Shiveluch, in Russia's Pacific coast Kamchatka peninsula, was ongoing on Monday night and that explosions sent ash up to 10 kilometres.
"Ongoing activity could affect international and low-flying aircraft," KVERT.
A large cloud of ash was drifting 340 kilometres to the north-north-west and 240 km to the west-south-west of the volcano, the geologists said.
The highest danger level, red, was declared for aviation.
KVERT warned that ash explosions up to 15 kilometres "could occur at any time."
The peninsula in the north Pacific Ocean has about 160 volcanoes, but only about two dozen of them are active.
UNESCO lists the Volcanoes of Kamchatka as a World Heritage Site.
