Russia and Belarus are extending military drills that were due to end on Sunday, the Belarus defense minister said, in a step that further intensifies pressure on Ukraine as Western leaders warn of an imminent Russian invasion.

The Belarus minister said the decision had been taken "in connection with the increase in military activity near the external borders" of Russia and Belarus and because of rising tension in the Donbass region of eastern Ukraine.

NATO says Russia has up to 30,000 troops in Belarus and could use them as part of an invasion force to attack Ukraine, although Moscow denies any such intention.