Russian President Vladimir Putin has announced snap military exercises on land, and in the Black and Caspian Seas, the defense ministry said in a statement.
The drills began at 7:00 a.m. (0400 GMT) Moscow time Thursday in Russia's southern, western and central military districts where troops have been put on combat alert, the statement said.
They will last until the end of the month and will involve various units and troops, from paratroopers to the Northern Fleet.
The latest military exercises come a week after Russia announced it was using an air base in Iran for bombing runs inside Syria. Russia has since withdrawn from the Iranian base. Also in the last week, President Putin verbally attacked Ukraine for allegedly employing its military intelligence to carry out acts of sabotage in Russia-annexed Crimea.
Poland reacted to Thursday's drills by immediately convening a briefing of the commands of its operational forces and military intelligence.
The United States and other governments in both western and eastern Europe have been increasingly concerned by Russia's military moves since 2014, when the Kremlin annexed Ukraine's Crimea region.
Estonia, Latvia and Lithuania have been particularly troubled about what they see as aggressive moves by Russia. The three smaller states asked NATO to expand its presence in the Baltic as a deterrent to Russia.
In May, Russia announced the deployment of three divisions of troops along its borders to counter NATO's increasing military presence in eastern Europe, a move that contributed to heightened tensions and anxiety in the region.
Source: Voice of America