"We are deadly serious in how we're going to face the threat that is currently being posed to both Ukraine and potentially to our security," Wallace said.

He was speaking ahead of talks in Brussels between NATO defence ministers and their counterparts from Ukraine and Georgia.

"This is not a joke or a light matter. This is a real challenge to the stability of Europe," Wallace added.

"One of the ways we can make sure there is no overspill or escalation is to provide resilience to our partners at NATO and that's what we're all doing."

The Western military alliance on Wednesday asked its military commanders to draw up plans to send more forces to its eastern members, amid persistent fears of a potential Russian invasion of Ukraine.

Wallace repeated accusations that Moscow was continuing to build up military deployments at its neighbour's border despite announcing the withdrawal of some troop.

"I think we have seen the opposite of some of the statements. We've seen an increase of troops over the last 48 hours -- up to 7,000.

"We've seen a bridge constructed from Belarus, into Ukraine or near Ukraine," he said.

"We'll take them at their word but we're going to judge Russia by their actions and at the moment, the troop build-up continues."

NATO Secretary General Jens Stoltenberg also reiterated the West had "so far... not see any sign of withdrawal or de-escalation".

NATO allies have rushed thousands of troops and hardware to bolster the alliance's eastern flank in response to Moscow's deployments around Ukraine.

The United States has said it is temporarily deploying around 4,700 additional soldiers to Poland.

Wallace said Britain had put an extra 1,000 troops on standby and was looking to double its contribution to NATO's battle group in Estonia.

NATO -- revitalised by the latest crisis after its catastrophic departure from Afghanistan -- is seeking to reassure nervous eastern allies in the longer-term.

The alliance is eyeing sending new battle groups to Romania and Bulgaria on the Black Sea. Deployments in Hungary and Slovakia have been mooted.

It is also looking at bolstering battle groups already in the Baltics and Poland -- currently totalling around 5,000 troops -- which were deployed in 2017 in response to Russia's seizure of Crimea.