Ukraine’s Russian-backed breakaway eastern territories have ordered military mobilizations amid a deadly escalation in fighting.
Men of fighting age in the self-declared people’s republics of Donetsk and Luhansk are being put on stand-by.
US President Joe Biden says he is convinced Russia will invade Ukraine, an allegation Moscow denies.
Western nations have accused Russia of trying to stage a fake crisis in the eastern regions as a pretext to invade.
International monitors report a “dramatic increase” in attacks along the line dividing rebel and government forces.
A Ukrainian soldier was killed by shelling on Saturday morning, the first such death reported in weeks.
Mr Biden’s Defence Secretary, Lloyd Austin, said Russian forces were beginning to “uncoil and move closer” to the border with Ukraine.
In the German city of Munich, US Vice-President Kamala Harris told a security conference that if Russia did invade, the US and its allies would impose a “significant and unprecedented economic cost”, targeting its financial institutions and key industries, as well as those who aided and abetted such an invasion.
Echoing her remarks, UK Prime Minister Boris Johnson said that, in the event of an invasion, his country would “open up the Matryoshka dolls” of strategic Russian-owned companies and make it impossible for them to raise finance in London.
Mr Johnson had talks in Munich with Ukrainian President Volodymyr Zelensky, who was visiting for the security conference against the advice of President Biden, who had said it might not be a “wise choice” for the Ukrainian leader to be out of his country at this time.
The US estimates there are 169,000-190,000 Russian personnel massed along Ukraine’s borders, a figure that includes separatist fighters in Donetsk and Luhansk.
Russian President Vladimir Putin, who oversaw major drills of Russia’s strategic nuclear missile forces from Moscow on Saturday, has said the situation in eastern Ukraine is deteriorating.
He said he remained willing to discuss the crisis with Western leaders, but accused them of ignoring Russia’s security concerns.
As we climbed the viewing platform, gale-force winds and hail storms were sweeping across the vast Obuz-Lesnovsky Military Range, around 100 miles (160km) from the border with Ukraine. Fluttering furiously, side by side, the flags of Belarus and Russia.
The skies cleared and an announcement came over the loudspeaker: “These are purely defensive drills.”
For the next half an hour, Russian and Belarusian forces rehearsed repelling an attacker, with tanks, fighter jets and rocket launchers. NATO considers this the largest Russian military deployment in Belarus since the Cold War. The numbers we’ve been hearing range from 10,000 to 30,000 Russian troops.
Ukraine views the exercises as part of Moscow’s “psychological pressure” on Kyiv. The drills are due to end on Sunday. When they do, a key question will be how quickly will the Russian soldiers pack up and go home?