Kyiv-bound, UK's Johnson vows to uphold Ukraine's sovereignty

British Prime Minister Boris Johnson will vow to uphold Ukraine's sovereignty on a visit to Kyiv on Tuesday as part of the West's diplomatic efforts to stop a possible Russian invasion which Moscow says there is no proof it is planning.

It comes as the United States said it is in active discussions with allies about possible US troop deployments to NATO's eastern flank, separate from some 8,500 forces already placed on alert last week.

Russia, which seized Crimea from Ukraine in 2014 and backs separatists in the east of the country, is demanding security guarantees including a promise NATO will never admit Kyiv.

The United States has said there is little chance of Ukraine joining soon but that the country should decide its own future, as Washington and Moscow clash over post-Cold War security arrangements in Europe and concerns over energy supplies.

Tensions were on display at the United Nations Security Council on Monday over the troop build-up near Ukraine as both Russia and the United States used the international forum to label each other as "provocative".

Johnson, who is facing calls to quit over gatherings held at his offices despite lockdown rules, is due to meet Ukraine's President Volodymyr Zelenskiy as he focuses on Britain's global role in the world, which he has much touted since Brexit.

"We urge Russia to step back and engage in dialogue to find a diplomatic resolution and avoid further bloodshed," he said in remarks released ahead of his arrival.

"As a friend and a democratic partner, the UK will continue to uphold Ukraine's sovereignty in the face of those who seek to destroy it."

Johnson is due to discuss with Zelenskiy what strategic support Britain can offer to Ukraine.

London has supplied defensive weapons and training personnel to Ukraine, though ministers have said that the deployment of combat troops is unlikely.

On Monday, the United States and Britain said they were prepared to punish Russian elites close to Russian President Vladimir Putin with asset freezes and travel bans if Russia enters Ukraine.

Poland has said it had offered neighboring Ukraine tens of thousands of munitions, and was awaiting a reply.

The United States ordered the family members of its government employees in Belarus to leave as it warned against travel there amid tensions over Ukraine.

Further diplomatic efforts are expected on Tuesday.

A call between Johnson and Putin, which had been planned for Monday, could take place on Tuesday, according to Downing Street.

French President Emmanuel Macron and Putin agreed during an exchange on Monday to maintain a dialogue on implementing the Minsk agreements regarding Donbass, a region of eastern Ukraine where Moscow has backed separatist fighters.

Russian Foreign Minister Sergei Lavrov is due to speak by phone with US Secretary of State Antony Blinken.

The State Department said on Monday it had received a written follow-up from Russia after Washington submitted responses last week to Moscow's demands over security arrangements on the continent.

Kremlin spokesman Dmitry Peskov had previously said that US and NATO statements describing Russia's main demands as unacceptable did not leave much room for optimism.

The United States said it would not comment publicly on the reply at this stage.

"It would be unproductive to negotiate in public, so we'll leave it up to Russia if they want to discuss their response," a State Department spokesperson said.