Speaking on Sky News' Trevor Phillips On Sunday, Mr Lewis said the "threats" Russia is directing at Ukraine are "inappropriate", adding that the UK and other western countries are "not going to put up with" them.

The Northern Ireland secretary also reiterated warnings that "an imminent incursion by Russia is entirely possible" after the country amassed 130,000 troops and heavy firepower along Ukraine's border.

Mr Lewis' comments came after Defence Secretary Ben Wallace warned it is "highly likely" that Vladimir Putin will order an attack on Ukraine, despite ongoing talks to avert a war.

Mr Wallace also compared diplomatic efforts to prevent a Russian invasion of Ukraine to appeasement.

Ukraine troops have been performing military drills amid fears of Russian invasion.

Troops and sanctions

There is no plan to send NATO forces into Ukraine in the event of a Russian attack as the country is not a member state.

But the alliance has made clear it would bolster NATO's eastern and southeastern flank with more troops to shore up its own defences.

Pressed on the fact that the UK has said it would not put troops on the ground in Ukraine, Mr Lewis said: "Well it may well be that President Putin is testing the boundaries of where he can go.

"And I think it is why it has been important that the West - and I have to say that the prime minister and the foreign secretary and the defence secretary, the UK as a whole - have been leading the way in bringing that alliance together to be very clear to Russia that this is not something that we will tolerate.

"We do want to see a peaceful outcome to this, but NATO is very, very clear that it is right that a democratic, independent country like the Ukraine is allowed to continue as a democratic, independent country without this kind of bullying or threats from a state like Russia."

Asked why the UK is not bringing in more sanctions on Russia amid fears of an invasion, Mr Lewis said: "We are not in that position yet."

Defence secretary warns invasion is 'highly likely'

Speaking in the Sunday Times, Mr Wallace said there is a "whiff of Munich in the air" - an apparent reference to the agreement that allowed the German annexation of the Sudetenland in 1938, but failed to prevent the Second World War.

And he warned that it was "worrying" that Russia's military build-up has continued despite high-level diplomatic talks increasing - prompting fears that the Kremlin is intent on invading Ukraine come what may.

British nationals in Ukraine are being urged by the Foreign Office to "leave now while commercial means are still available" - with Armed Forces minister James Heappey warning that the RAF would not carry out evacuations should war break out.

Israel, Portugal and Belgium have also ordered their citizens to leave, while Australia has begun evacuating its embassy in Kyiv.

Biden speaks to Putin amid rising tensions

On Saturday, US President Joe Biden spoke to his Russian counterpart Mr Putin for over an hour - warning America and its allies will "respond decisively" if there is an invasion.

Talks have gained a sense of urgency after US intelligence suggested that the Kremlin could take action before the end of the Winter Olympics in Beijing on 20 February - far sooner than analysts had expected.

The Russian president told Mr Biden that Washington's response to Moscow's security demands had not taken into account key concerns, and the West had not put enough pressure on Ukraine to abide by the Minsk agreements.

A senior official in the White House described the call as professional and substantive, but that there was no fundamental change.

Russia accuse US of stoking 'hysteria'

Maria Zakharova, a spokeswoman for Russia's Foreign Ministry, has accused Joe Biden's administration of stoking "hysteria".

Ukrainian President Volodymyr Zelenskyy also sought to downplay the threat, saying: "The best friend of our enemies is panic in our country. And all this information is just provoking panic and can't help us."

On Saturday, thousands of Ukrainians marched through the centre of Kyiv - chanting "Glory to Ukraine" and carrying banners that said "Ukrainians must resist" and "invaders must die".

Although Mr Zelenskyy has urged Ukrainians to remain calm, he agrees with Washington's assessment that a Russian attack could happen at any time, and attended police drills in the southern region of Kherson.

This has become the biggest security crisis between Russia and the West since the Cold War, and US officials believe they have a matter of days to prevent an invasion that could cause enormous bloodshed in the region.