Russian-backed separatist leaders in eastern Ukraine have declared a full military mobilisation, a day after ordering women and children to evacuate to southern Russia because of what they said was the threat of conflict.

Denis Pushilin, head of the self-proclaimed Donetsk People's Republic, said in a video statement that he had signed a decree on mobilisation and called on men "able to hold a weapon in their hands" to come to military enlistment offices.

Another separatist leader, Leonid Pasechnik, signed a similar decree for the Luhansk People's Republic shortly afterwards.

Separatist authorities earlier announced plans to evacuate about 700,000 people to neighbouring Russia, citing fears of an imminent attack by Ukrainian forces — an accusation Kyiv flatly denied.

"We are fully committed to diplomatic conflict resolution only," Ukraine's Foreign Minister, Dmytro Kuleba, tweeted.

Metadata from two videos posted by the separatists announcing the evacuation show that the files were created two days ago, according to the Associated Press.

US authorities have alleged that a disinformation campaign by the Kremlin could include staged, prerecorded videos.

Authorities began moving children from an orphanage in Donetsk, and other residents boarded buses for Russia.

Long lines formed at petrol stations as more people prepared to leave on their own. 

Russian President Vladimir Putin ordered the government to offer a payment of 10,000 roubles ($180) to each evacuee, equivalent to about half of an average monthly salary in the war-ravaged Donbas region.

Less than 7,000 people had been evacuated from Donetsk as of Saturday morning, the local emergencies ministry said.

Ukrainian forces have been fighting pro-Russia rebels in eastern Ukraine since 2014, in a conflict that has killed some 14,000 people.

With an estimated 150,000 Russian troops now posted around Ukraine's borders, the long-simmering separatist conflict could provide the spark for a broader attack.

The Ukrainian military said it had recorded 12 ceasefire violations by pro-Russian separatists in eastern Ukraine in the morning after 66 cases in the previous 24 hours.

Separatist authorities also reported what they said was shelling by Ukrainian forces of several villages.

Both sides regularly trade blame for ceasefire violations.

Monitors from the Organization for Security and Cooperation in Europe reported more than 600 explosions in the war-torn east of Ukraine on Friday.

In a further indication Russia could be preparing for a major military push, a US defence official said an estimated 40 per cent to 50 per cent of the ground forces deployed in the vicinity of the Ukrainian border had moved into attack positions closer to the border.

But other officials have said that shift has been underway for about a week, and does not necessarily mean Mr Putin has decided to begin an invasion.

The Russian defence ministry said Mr Putin would oversee exercises by Russia's nuclear forces involving the launch of ballistic and cruise missiles.

Mr Putin pledged to protect Russia's national interests against what it sees as encroaching Western threats.

A senior US official called the decision "escalatory and unfortunate".

Asked about Western warnings of a possible Russian invasion on Wednesday that did not materialise, Mr Putin said: "There are so many false claims, and constantly reacting to them is more trouble than it's worth."

"We are doing what we consider necessary and will keep doing so," he said.

"We have clear and precise goals conforming to national interests."

Kamala Harris to meet Ukraine President

UN Secretary-General Antonio Guterres said the threat to global security was "more complex and probably higher" than during the Cold War.

He told a security conference in Munich that a small mistake or miscommunication between major powers could have catastrophic consequences.

US Vice-President Kamala Harris is expected to meet Ukrainian President Volodymyr Zelenskyy in Munich on Saturday (local time), in what is likely to be a powerful display of diplomatic support for the European nation.

It comes a day after US President Joe Biden said he was "convinced" Russia had made a decision to invade Ukraine.

Ms Harris earlier met NATO Secretary-General Jens Stoltenberg and the leaders of Estonia, Latvia and Lithuania, and said Russia must show it was open to diplomacy, while heralding unity in the 30-member NATO alliance and warning Moscow of severe consequences if it invaded Ukraine.

Ms Harris declared "our greatest strength is our unity" as she met with the Baltic leaders.

The Baltic countries have requested the US increase its troop presence on the eastern edge of the NATO region.

"This is a moment that has made that clear: that our unity is evidence and is a measure of our strength," Ms Harris said.

Mr Biden said Mr Putin had decided to invade Ukraine, and he continued to spread false information to try to build a pretext for a military strike that could happen in days.

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Russia wants to stop Kyiv from joining NATO and accuses the West of hysteria.

Asked if the United States was concerned about whether Mr Zelenskyy should travel to Munich for meetings Mr Biden said: "That's a judgement for him to make."

A statement from Mr Zelenskyy's office said he would attend the Munich Security Conference and return home later the same day.

Mr Zelenskyy has long said that, while he believes Russia is threatening to attack his country, the likelihood of an imminent invasion has been overstated by Western allies, responding to Moscow's efforts to intimidate Ukraine and sow panic.

Russia announced this week it was pulling back forces from vast military exercises, but US officials said they saw no sign of a pullback and instead observed more troops moving toward the border with Ukraine.

The White House and the UK formally blamed Russia for recent cyber attacks targeting Ukraine's defence ministry and major banks.

Russia rejected the accusations.

"There is still a chance to avoid unnecessary bloodshed, but it will require an overwhelming display of Western solidarity beyond anything we have seen in recent history," British Prime Minister Boris Johnson said in a statement ahead of his visit to Munich.

"Allies need to speak with one voice to stress to President Putin the high price he will pay for any further Russian invasion of Ukraine. Diplomacy can still prevail."

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Reuters/AP