Defence Secretary Ben Wallace is also travelling to Eastern Europe to meet officials ahead of an announcement on the deployment of British troops to the region. He is expected to travel to Hungary before heading to Slovenia and Croatia.

Meanwhile, Foreign Secretary Liz Truss, who is visiting Ukraine this week before heading to Moscow next week for talks, has told Sky News that she is "ruling absolutely nothing out" when it comes to UK support for Ukraine.

She said Russian oligarchs will have "nowhere to hide" under toughened UK sanctions, and British weapons and cash could be sent to Ukraine if Russia invades.

Speaking to Sky News' Trevor Phillips On Sunday - amid growing tensions between the West and Moscow over Russia's build-up of troops on Ukraine's border - Ms Truss said: "The number one priority is deterring Vladimir Putin from taking that action.

"That's why we are offering to deploy extra troops into Estonia, we are providing more air support across the Black Sea, and we are supplying defensive weapons to Ukraine to make sure they are in the best possible position should Vladimir Putin try to stage an incursion.

"What I'll be announcing later this week is improved legislation on sanctions so we can target more Russian interests that are of direct relevance to the Kremlin.

"Because we absolutely need to stop this happening, that is our number one priority."

Prime Minister Boris Johnson warned on Sunday of an "increasingly concerning" situation over Ukraine, after he received briefings this week from defence chiefs.

"I continue to urge Russia to engage in negotiations and avoid a reckless and catastrophic invasion," he posted on Twitter.

Mr Johnson is looking at doubling the number of UK troops deployed to strengthen Europe's borders in a new offer to NATO, as well as sending fast jets, warships and military specialists.

UK 'ruling absolutely nothing out' in assisting Ukraine - but British troops 'unlikely'

In her appearance on Sky News, Ms Truss said she was "ruling absolutely nothing out" when it comes to the UK's support for Ukraine, as she was asked if Britain might supply money and weapons to Ukrainian forces in the event of a Russian invasion.

However, the foreign secretary has deemed it "very unlikely" that British troops could be sent to fight alongside Ukrainians against Russia.

NATO secretary-general Jens Stoltenberg has also said there are "no plans" to deploy NATO combat troops to Ukraine, in the event of a Russian invasion.

He noted there was "a difference" between being a full NATO member, with a mutual defence obligation between countries within the alliance, and a "strong and highly valued partner" such as Ukraine.

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Ukraine's plans for future membership of NATO has been cited as one of the key reasons behind the current tensions between Moscow and the West.

Russia's foreign minister Sergey Lavrov on Sunday claimed NATO wants to pull Ukraine into the alliance, telling his country's state television that NATO "has already come close to Ukraine".

"They also want to drag this country there," he added. "Although everyone understands that Ukraine is not ready and could make no contribution to strengthening NATO security."

Meanwhile, the head of Russia's security council, Nikolai Patrushev, rejected Western warnings about a planned invasion of Ukraine.

"At this time, they're saying that Russia threatens Ukraine - that's completely ridiculous," he was quoted as saying by state news agency Tass.

"We don't want war and we don't need it at all."

Russian oligarchs warned over London property under new sanctions

In a warning to those close to Mr Putin, Ms Truss said of the widening of UK sanctions laws: "Any company of interest to the Kremlin and the regime in Russia would be able to be targeted.

"So there would be nowhere to hide for Putin's oligarchs or Russian companies involved in propping up the Russian state.

"That is what we are looking at doing this week."

Ms Truss did not rule out the possibility that tougher sanctions, expected to be outlined in parliament on Monday, could include the seizure of London property of Russian oligarchs in the UK.

"Nothing is off the table," the foreign secretary said. "We're not going to go into the details of exactly who and how we would target the sanctions.

"What the legislation enables us to do is hit a much wider variety of targets, so there can nobody who thinks they would be immune to those sanctions."

More on this story:
Why is Russia worried about NATO - and what does it have to do with Ukraine?

Germany challenged over Nord Stream 2 project

Ms Truss said the UK was working "very closely" with allies such as the US and EU on a "severe" package of sanctions to make it "very, very difficult" for Russia.

And, in a message to Berlin, Ms Truss also said that a new gas pipeline between Russia and Germany should be halted in the event of Mr Putin invading Ukraine.

"We cannot favour short-term economic interests over the long-term survival of freedom and democracy in Europe," she said.

"That's the tough decision all of us have got to make."

Asked whether the UK's view was that the Nord Stream 2 project should stop completely in such a scenario, Ms Truss replied: "Completely."

Putin facing 'quagmire' in Ukraine

The foreign secretary also told Russia that it faced "a quagmire" like the Kremlin's previous conflicts in Afghanistan and Chechnya, as she warned that "the Ukrainians will fight hard, they are not going to give up easily".

She said the UK's role was to "make sure" that a military invasion of Ukraine would not be "simple" for Russia.

Labour accuse ministers of 'completely contradictory' foreign policy

Labour's shadow levelling up secretary Lisa Nandy accused the government of a "completely contradictory" approach in its foreign policy.

She compared the UK's desire to work with EU allies over Russia with ministers' continuing row with Brussels over Brexit arrangements in Northern Ireland.

Ms Nandy told Sky News the government "really ought to reassess" its approach to issues with the Northern Ireland Protocol "in the light of this very serious situation that's developing on the border of Ukraine".