America’s Vice President Kamala Harris has warned Russia will face a “swift, severe and united” retaliation and suffer “great damage” if it invades Ukraine as the US suspects is certain.

Speaking at the Munich Security Conference where world leaders are discussing international security, Ms Harris issued a public warning to the Kremlin.

It comes as Russian President Vladimir Putin oversaw the testing of ballistic and cruise missiles and as rebels backed by Moscow declared a full military mobilisation in eastern Ukraine.

Russian-backed separatists have issued a call to men “able to hold a weapon in their hands” to enlist as violence flared in eastern Ukraine where the rebels have control.

Ms Harris warned that America would hit back with economic sanctions if Russia attempted to move Ukraine’s borders by force.

“We have prepared, together, economic measures that will be swift, severe, and united,” she said.

“We will impose far-reaching financial sanctions and export controls. We will target Russia’s financial institutions and key industries.

“And we will target those who are complicit and those who aid and abet this unprovoked invasion.

“Make no mistake: The imposition of these sweeping and coordinated measures will inflict great damage on those who must be held accountable.

“And we will not stop with economic measures. We will further reinforce our NATO Allies on the eastern flank.”

Kamal Harris addresses the Munich Security Conference. Photo: Getty

However Ukraine’s President Volodymyr Zelensky called for sanctions to happen now to avert any potential invasion or war.

“We don’t need your sanctions after the bombardment will happen and after our country will be fired at or after we will have no borders, or after we will have no economy … why would we need those sanctions then?” he told CNN.

Venting his frustration, Ukrainian President Volodymyr Zelenskiy said the global security architecture was “almost broken”. He urged the permanent members of the UN Security Council, Germany and Turkey to meet to draw up new security guarantees for his country.

“The rules that the world agreed on decades ago no longer work,” Zelenskiy said. “They do not keep up with new threats … This is a cough syrup when you need a coronavirus vaccine.”

World Bank President David Malpass told Zelenskiy on Saturday the bank was readying funding to Ukraine of up to $US350 million ($A488 million).

Rumours of war

Western leaders were meeting amid reports of explosions just inside Russian territory to Ukraine’s east and in the parts of eastern Ukraine that are controlled by Moscow-backed rebels.

They fear a conflict on a scale unseen in Europe at least since the Yugoslav and Chechen wars of the 1990s, when hundreds of thousands died and millions fled, and have urged Mr Putin to negotiate.

German Foreign Minister Annalena Baerbock said the West had agreed on tailor-made sanctions packages that were ready to go if necessary.

But Ms Baerbock said there was still time to avert a major military conflict.

“History has not yet been written: there is an exit that the Russian government can choose at any time,” said Ms Baerbock.

“Our common message to them is very clear: Don’t make this fatal mistake. Withdraw your troops … Let’s talk.”

Chinese Foreign Minister Wang Yi, whose country and Russia are diplomatically aligned, had harsh words for all sides in the dispute, seeing a revived “Cold War mentality” in the confrontation.

He said no country, not even a superpower, should replace international norms with its own will.

But British Prime Minister Boris Johnson drew a parallel between Russia’s intentions towards Ukraine and China’s towards Taiwan, arguing that Western leaders had a duty to be firm.

“If Ukraine is invaded, the shock will echo around the world. And those echoes will be heard in east Asia and will be heard in Taiwan,” he said.

“People would draw the conclusion that aggression pays, and that might is right.”

Russia’s President Vladimir Putin is set to oversee exercises by strategic nuclear missile forces as Western leaders gather in Munich, fearful he could order troops massed on Ukraine’s border to invade at any time.

Military mobilisation

Civilians take part in a military training course conducted by a Christian Territorial Defence Unit in Ukraine. Photo: Getty

Russian-backed separatist leaders in eastern Ukraine declared a full military mobilisation, citing the threat of an imminent attack by Ukrainian forces.

Kyiv flatly denied the accusation and Washington said it was part of Russia’s plan to create a pretext for an invasion of Ukraine.

Multiple explosions could be heard on Saturday morning in the north of the separatist-controlled city of Donetsk in eastern Ukraine, a Reuters witness said.

The origin was not immediately clear. Ukraine said earlier that one of its soldiers had been killed.

Russia ordered the military build-up while demanding NATO stop Kyiv ever joining the alliance but says predictions it is planning to invade Ukraine are wrong and dangerous.

It says it is now pulling back while Washington and allies insist the build-up is continuing in one of the worst crises since the Cold War.

US President Joe Biden, who has given regular warnings of an impending invasion, said on Friday he now believes the capital Kyiv would be targeted by Russia but that he does not think Mr Putin is even remotely contemplating using nuclear weapons.

“We have reason to believe the Russian forces are planning to and intend to attack Ukraine in the coming week, in the coming days,” Mr Biden told reporters at the White House.

“As of this moment, I am convinced that he has made the decision.”

Meanwhile analysts said Russia’s missile testing was a show of strength.

Russia has released footage to show it is withdrawing troops from the border but the United States says there has been a ramp-up to between 169,000-190,000 troops, from 100,000 at the end of January.

The Kremlin also has tens of thousands of troops staging exercises in Belarus, north of Ukraine, that are due to end on Sunday.

Belarus’ leader Alexander Lukashenko said on Friday they could stay as long as needed.

-with AAP