The volcano in the Russian Far East is the highest mountain in Kamchatka and the tallest active volcano in Europe and Asia.
Klyuchevskoi has been erupting off and on since August, RIA Novosti reported, and the Emergency Services Ministry said it just produced an ash cloud almost 4 miles high.
"We have observed the latest ash eruption from the Klyuchevskoi volcano," local officials with the ministry said in a statement. "The ash cloud is moving in a northeast direction."
In addition to issuing the aviation warning, the ministry has advised tour companies to avoid the area around the volcano.
Klyuchevskoi erupted in 2005, 2008, 2010 and 2012.