Russian President Vladimir Putin has ordered troops in central Russia on "full combat alert," the defence minister said on Saturday, a day after the Kremlin confirmed it was beefing up its military presence at the border with Ukraine.

"In accordance with his (Putin's) order, from 11:00 am Moscow time (0700 GMT) the troops of the central military district as well as formations and military units located on its territory have been put on full combat alert," Russian news agencies quoted Defence Minister Sergei Shoigu as saying.