Britain has been in talks with the US about potential deployments as concern mounts over what action Russian President Putin may take as he continues to mass tens of thousands of troops and weaponry around Ukrainian borders.

A Whitehall source stressed that no decision had yet been taken but that sending hundreds more British forces to "eastern Europe" was an option - "one of many".

A second Whitehall source said nothing had been signed off but confirmed a new deployment was possible, though they said it was still "very much speculation".

Such a move could be used as a last-ditch effort to make the Kremlin think twice if diplomatic efforts to defuse the crisis fail because it would be the exact opposite of what Mr Putin has demanded.

He wants all NATO forces to withdraw from eastern and central European countries, saying their presence poses a security threat to Russia. NATO denies the claim.

It would also bolster protection for nervous eastern NATO member states that worry they could be next to face a Russian attack should President Putin decide to go on the offensive again in Ukraine.

Prime Minister Boris Johnson has already signalled that the UK would consider increasing its forces in the east of Europe in response to any invasion of Ukraine.

The United States has also put 8,500 troops on a heightened alert to mobilise to Europe to support NATO if needed.

The alliance has started to strengthen its defences with a small number of warplanes and ships. It has also signalled it was preparing to increase its forces along its eastern flank should Russia launch a new assault on Ukraine.

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CNN reported on Wednesday that the US and a handful of allies, including the UK, were in discussions to send thousands more personnel to eastern European NATO countries ahead of any potential further move into Ukraine by Russia.

Ukraine is not one of the alliance's 30 members but has said it would like to join - a desire that is one of the major flashpoints of friction between the West and Moscow.

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NATO mulls new battlegroups

Sky News understands that NATO allies have been mulling the possibility of setting up new 1,000-strong battlegroups in Romania, Bulgaria, Hungary and Slovakia.

France has offered to lead any new battlegroup in Romania

They would be similar to so-called Enhanced Forward Presence deployments in the three Baltic states and Poland.

These were founded in 2016 as part of NATO's move to bolster its defences and ability to deter Russian aggression after the Crimea annexation in 2014 and its backing of separatists in eastern Ukraine.

The UK leads the Estonia battlegroup, contributing about 800 servicemen and women. It also contributes some 150 troops to a US-led battlegroup in Poland.

All 30 NATO members need to agree

The CNN report said the new mobilisations under consideration by the US and the small number of allies included possible deployments of approximately 1,000 military personnel each to Romania, Bulgaria and Hungary, similar to the existing battlegroups in the Baltics and Poland.

It was not immediately possible to confirm this detail.

A new deployment would only be a NATO mission if all 30 members agree to it - even if they do not contribute troops.

The Telegraph first reported on the potential new British deployment. It quoted government sources as saying that "very advanced discussions" were under way.