Moscow is sending troops from the country's far east to Belarus for drills, a deployment that will increase the number of military forces positioned near Ukraine's borders.

Germany signalled on Tuesday that it could halt the Nord Stream 2 pipeline from Russia if Moscow invades Ukraine, and western nations have rallied behind Kyiv.

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Prime Minister Boris Johnson told his Cabinet on Tuesday that Russian President Vladimir Putin "must not be allowed to rewrite the rules" of international relations as tensions continued to mount.

The UK has promised anti-tank weapons to Ukraine in response to a Russian military build-up on the border, with around 100 British troops involved in a training operation.

Asked about the pipeline after meeting NATO Secretary-General Jens Stoltenberg, Germany's Chancellor Olaf Scholz said it was "clear that there will be a high price to pay and that everything will have to be discussed should there be a military intervention in Ukraine".

Meanwhile, US Secretary of State Antony Blinken will meet Ukrainian President Volodymyr Zelenskiy on Wednesday, before heading to Berlin to discuss "joint efforts to deter further Russian aggression against Ukraine" with German, British and French officials.

He will then meet Russian foreign minister Sergei Lavrov in Geneva on Friday.

White House press secretary Jen Psaki said the situation is "extremely dangerous", adding that: "We're now at the stage where Russia could, at any point, launch an attack in Ukraine."

Russia denies any plans to attack but said it could take unspecified military action unless its demands - including a promise by the NATO alliance to never admit Ukraine - are met.

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Russian deputy defence minister Alexander Fomin said the war game drills taking place in Belarus will involve exercises to "thwart and repel foreign aggression". He did not say how many troops and weapons are being redeployed for the exercise, or give the number of troops that will participate.

Former Ukrainian president Petro Poroshenko said the "whole world is very much disturbed by the possibility of Russian aggression", adding that the country should follow the example set by the Conservative and Labour parties in the UK during World War Two and "demonstrate this unity in Ukraine".

He said he blames the current president for failing to "mobilise" and unite the nation.

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Mr Poroshenko returned to the country on Monday to face court on treason charges being pursued by President Zelenskiy.

Mr Poroshenko is facing claims that he was involved in the sale of large amounts of coal that helped finance Russian-backed separatists in eastern Ukraine in 2014-15.

He maintains his innocence and claims his successor is trying to discredit him to detract from the ongoing problems in the country.