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.

The latest assessments from U.S. and Ukrainian analysts say more than 100,000 Russian troops are deployed on the Ukrainian border.