Ukraine’s Russian-backed breakaway eastern territories have ordered military mobilisations amid an escalation in fighting.
Men of fighting age in the self-declared people’s republics of Donetsk and Luhansk are being put on stand-by.
Monitors report a “dramatic increase” in attacks along the line dividing rebel and government forces.
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 crisis in the breakaway regions to give it a reason to launch an offensive.
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 is expected to oversee major drills of Russia’s strategic nuclear missile forces 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.
In the German city of Munich, Ukrainian President Volodymyr Zelensky is due to meet Western leaders including US Vice-President Kamala Harris at a security conference.
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.
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.
Decreeing the mobilisation, the head of the Donetsk People’s Republic, Denis Pushilin, called on “all men capable of bearing arms to rise to the defence of their families, children, wives and mothers”.
“Through our joint efforts, we will achieve the victory we desire and need,” he wrote on Telegram. “We will defend the Donbas and all Russian people.”
In Luhansk, separatist leader Leonid Pasechnik forbade all men aged 18-55 to leave the territory and said the authorities there reserved the right to requisition vehicles and other property for the “needs of the defence”.
A number of civilians have been evacuated from the rebel territories to neighbouring Russia, with at least 6,500 leaving Donetsk, according to the separatists.
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”.
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.
Its latest daily report – for Thursday – registered 870 ceasefire violations, including 654 explosions. It also confirmed a kindergarten had been damaged earlier in the week in government-controlled territory.
Late on Friday, President Biden said he believed Mr Putin had decided to launch an invasion of Ukraine.
“As of this moment I am convinced that he has made the decision,” he said. Previously, the president and his top officials have said they did not know whether this was the case.
- EXPLAINER: What is a false flag attack?
- CONTEXT: Satellite images show Russian activity near Ukraine
- BACKGROUND: Is Russia going to invade Ukraine?
UK Prime Minister Boris Johnson has urged other Western leaders to unite in an “overwhelming display of Western solidarity” to avoid bloodshed in Ukraine.
Speaking before he left for Munich, he said they needed to speak to President Putin “with one voice” to stress “the high price he will pay for any further Russian invasion of Ukraine”.