The Shiveluch volcano began sputtering shortly after a powerful 7.0 magnitude earthquake struck off Kamchatka's east coast early Sunday, according to volcanologists from the Russian Academy of Sciences.
They warned that another, even more potent earthquake may be on the way.
The academy's Institute of Volcanology and Seismology released a video showing the ash cloud over Shiveluch.
It stretched over 490 kilometres east and southeast of the volcano.
The Ebeko volcano located on Kuril Islands also spewed ash 2.5 kilometres high, the institute said.
It did not explicitly say whether the earthquake touched off the eruptions.
A "code red" ash cloud warning briefly put all aircraft in the area on alert, the Kamchatka Volcanic Eruption Response Team reported.
A separate report on Sunday carried by the official Tass news agency said that no commercial flights had been disrupted and there was no damage to aviation infrastructure.
The tremors in the area may be a prelude to an even stronger earthquake in south-eastern Kamchatka, Russian scientists warned.
The Institute of Volcanology said a potential second quake could come "within 24 hours" with a magnitude approaching 9.0.
There were no immediate reports of injuries from Sunday's earthquake, which struck at a depth of 6 kilometres under the sea bed with the epicentre 108 kilometres southeast of the nearest city, according to Russian emergency officials.
Russian news outlets cited residents of Petropavlovsk-Kamchatsky, a port city of more than 181,000 people that sits across a bay from an important Russian submarine base, reporting some of the strongest shaking "in a long time".
On November 4, 1952, a magnitude 9.0 quake in Kamchatka caused damage but no reported deaths despite setting off 9.1-metre waves in Hawaii.
