Two volcanoes erupted Thursday on Russia's far-eastern Kamchatka Peninsula, tossing massive ash clouds kilometres into the air, forcing flights to divert and blanketing one town with thick ash.

The Klyuchevskaya Sopka, Eurasia's highest active volcano, exploded along with the Shiveluch volcano, 70 kilometres to the northeast, the Russian Emergency Situations Ministry's branch in Kamchatka said, adding that flights in the area had to change course.

Ash clouds from the remote Siberian volcanoes billowed up to 10 kilometres and were spreading east across the Pacific Ocean, vulcanologist Sergei Senyukov told Rossiya 24 television. Streams of lava flowed down the slopes of Shiveluch.

U.S. Federal Aviation Administration spokeswoman Tammy Jones said the agency does not anticipate any effects from the eruptions on air traffic to the United States.

The Volcanic Ash Advisory Centre in Tokyo issued an advisory for planes to be on alert for the ash cloud, although Tokyo's Narita airport said it had no flights diverted yet.

Volcanic ash blanketed the nearby town of Ust-Kamchatsk, reducing visibility to only a few metres and turning buildings ghostly white. Emergency officials said the town's 5,000 residents weren't in any immediate danger, but urged them to stay indoors, and tightly close doors and windows to avoid inhaling ash particles that could lead to respiratory illnesses and allergic reactions.

Schools and businesses in Ust-Kamchatsk quickly closed and all streets were shut to traffic. Scientists warned that ashes will likely continue falling on the area for at least 10 days.

Ust-Kamchatsk is 70 kilometres east of Shiveluch and 120 kilometres northeast of Klyuchevskaya Sopka, and winds blew ash from both on the town.

Ash moving across Bering Sea

Shiveluch quieted down later Thursday, but Klyuchevskaya Sopka, which stands 4,750 metres high, kept erupting, Russian officials said.

Jen Burke, a meteorologist with the Alaska Aviation Weather Unit, said ash from the Shiveluch eruption — the larger of the two — was moving across the Bering Sea at a height of 7,620 metres. That could put it in the path of planes flying between Asia and North America over Alaska.

"Right now it's not a difficult area to avoid because it's north of the Aleutian Islands," Burke said. "Planes could fly south of the Aleutian Islands and be perfectly safe."

She said ash might affect the extreme west coast of Alaska, but winds were predicted to push the cloud north. 

Kamchatka, which juts into the Pacific, is studded with active volcanoes.

The Emergency Situations Ministry warned Thursday that another volcano across the peninsula to the south, Gorely, has begun spewing gases and could erupt any moment. Gorely is about 70 kilometres south of Kamchatka's regional capital, Petropavlovsk-Kamchatsky.

Kamchatka volcanoes are part of the Ring of Fire string of volcanoes encircling the Pacific. It wasn't immediately clear whether the volcanic activity on Kamchatka was related to Tuesday's eruption of the Mount Merapi volcano in Indonesia that killed 33.