Ukraine's Russian-backed breakaway eastern territories have ordered military mobilisations amid a deadly escalation in fighting.
A Russian tank near the border with Ukraine Photo: AFP
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.
Two Ukrainian soldiers were killed and four injured by shelling on Saturday, the first deaths to be reported in weeks.
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.
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.
Ukraine, a former Soviet republic with historic ties to Russia, is not a member of NATO or the European Union but has close relations with both.
Russia is insisting Ukraine should not be allowed to join NATO, which it sees as a threat to its security.
Accurate figures are difficult to establish but as many as 3.5 million people live in the two rebel territories, which broke away in 2014 after Ukraine's pro-Russian government was overthrown. Since then, at least 720,000 have acquired Russian citizenship, according to Russian media.
A tent camp for people evacuated from Donetsk, in the Rostov region, Russia. Photo: Sputnik via AFP
A number of civilians have been evacuated from the rebel territories to neighbouring Russia, with at least 6500 leaving Donetsk, according to the separatists.
In Luhansk, the first train to Russia since 2014 left with 650 evacuees aboard, Russian media report.
One person in rebel-controlled Luhansk told BBC News on condition of anonymity that the separatists were trying to stir panic deliberately and said local people were wary of evacuating.
Another in rebel-controlled Donetsk confirmed that some people were leaving and added, "People just want that somebody finally can take control and responsibility for these separatist-held territories whoever it would be - Russia or Ukraine".
Details of the Ukrainian soldier's death were not given but it was contained in a report that spoke of artillery, mortar and grenade attacks by the separatists.
There were also reports of attacks or attempted attacks inside separatist-held territory which could not be independently verified:
- An unexploded booby-trapped car was discovered on a road being used to bus evacuees from Luhansk to Russia, separatist security sources told Russian media
- Two overnight blasts targeting a gas pipe in Luhansk were blamed on Ukrainian saboteurs
- According to Russian news agency Interfax, the separatists in Donetsk say a number of people were killed or injured as a result of acts of sabotage by Ukrainian forces. No details were given
In another development, two shells reportedly exploded on Russian territory in Rostov region, about a kilometre from the Ukrainian border. Russia opened a criminal investigation.
Ukrainian Foreign Minister Dmytro Kuleba denied Russian shells had hit Russian territory and called for an "immediate and impartial international investigation of the incidents reported by Russian media".
International observers from the Organization for Security and Co-operation in Europe have been monitoring a truce between the separatists and government forces, who have fought a bloody war at a cost of at least 14,000 lives.
On Friday, the observer mission reported "a dramatic increase in kinetic activity along the contact line in eastern Ukraine, equal to the numbers of ceasefire violations reported" before July 2020.
- BBC