Russia announced on Monday it was carrying out new military exercises close to the border with Ukraine.
More than 100 bombers and fighter jets will take part in the manoeuvres throughout the week in Russia’s central and western regions.
Last month Russian President Vladimir Putin promised an “adequate and well-measured response” to NATO building up its forces in Eastern Europe, including Poland, the Black Sea and Baltic Sea.
NATO’s military exercises have been described as ‘provocative’ and are viewed by Moscow as an attempt to expand towards Russia’s borders.
Meanwhile more than 400 Ukrainian troops are understood to have asked for asylum in Russia.
Fighting in eastern Ukraine has intensified, with Kiev stepping up a campaign against rebels in their two main strongholds of Donetsk and Luhansk. Moscow denies supplying the rebels with weapons.