In his third conversation with the Russian president in two months,  Biden said Washington would respond "decisively and impose swift and severe costs" if Russia moved into Ukraine.

Western estimates put at about 30,000 the number of Russian troops taking part in the exercises – along with most of the Belarusian army.

The country has also brought the redemption of gross domestic product (GDP) warrants to 20 percent.

Fitch said there was a high degree of uncertainty about the near-term strategic objectives of President Putin, and the increased risk of an invasion reflected the extended period of troop build-up along Ukraine's border.

Lukashenko’s support for Moscow stands in contrast with the neutrality he sought to adopt eight years ago when Russia seized the Crimean Peninsula from Ukraine and fomented a separatist revolt in Ukraine’s eastern Donbas region.