UK ministers last week gained powers to impose tough new sanctions on Russian oligarchs and businesses.

But Mr Lavrov, who last week held a stormy meeting with Foreign Secretary Liz Truss, has vowed Moscow will respond to any action taken by London.

New Russia sanctions 'ready to go'

Prime Minister Boris Johnson has said the "very, very tough" package of sanctions is "ready to go" should Russia launch a "reckless" invasion of Ukraine.

The Foreign Office said legislation laid in parliament last week would allow the UK to sanction those linked directly to Russia's agitation over Ukraine, as well as Kremlin-linked organisations and businesses of "economic and strategic significance" to the Russian government.

This includes their owners, directors and trustees.

The US and EU have also threatened increased sanctions against Russia should it invade Ukraine.

Caution urged over Russian claims of withdrawal

Defence Secretary Ben Wallace on Wednesday urged caution over Russian claims it was withdrawing some of its estimated 150,000 troops from near Ukraine's borders.

The Russian defence ministry released a video showing a trainload of armoured vehicles moving across a bridge away from Crimea, the Black Sea peninsula that Russia annexed from Ukraine in 2014.

It said the movement was part of a return of forces to their permanent bases, which Moscow has claimed started on Tuesday.

However, there has been scepticism over Russia's claims of withdrawal from among NATO allies.

Mr Wallace, who attended a meeting of NATO defence ministers in Brussels on Wednesday, told Sky News: "What we haven't seen is evidence of withdrawal that has been claimed by the Kremlin.

"Until we see a proper de-escalation we should all be cautious about the direction of travel from the Kremlin."

There was also concern over what was described as the largest cyberattacks of their kind on Ukraine, which downed the web portal of the country's defence ministry and disrupted some banking services.

Ukraine said Russia was likely behind the distributed denial of service (DDoS) attacks, although the Kremlin denied involvement.

NATO chief warns of 'new normal' in Europe

NATO secretary general Jens Stoltenberg also said there was no sign of Russian military de-escalation, but added it was "not too late for Russia to step back from the brink of conflict".

Speaking at a news conference in Brussels after the defence ministers' meeting, Mr Stoltenberg said there was some "cautious optimisim" that Russia had "indicated their willingness to engage in diplomatic efforts".

But he added: "What we see on the ground is no withdrawal of troops, forces and equipment.

"Actually what we see is Russian troops are moving into position and we saw the cyberattack.

"These are the kinds of actions and measures that we expect will come in advance of a bigger military intervention into Ukraine.

"So of course this is of concern and that's the reason why we continue to call on Russia to deescalate."

Mr Stoltenberg expressed his regret at a "new normal in Europe" whereby Russia "has made it clear that it is prepared to contest the fundamental principles that have underpinned our security for decades, and to do so by using force".

"We do not know what will happen in Ukraine, but the situation has already demonstrated that we face a crisis in European security," he said.

The NATO chief revealed the alliance's defence ministers had agreed to establish new NATO battle groups in central, eastern and south-eastern Europe.

France has offered to lead such a battle group in Romania, Mr Stoltenberg said.

'Invasion would shake the foundations of international order'

The UK government has highlighted the establishment of Russian field hospitals and blood banks, as well as the movement of strategic weapons systems, as contradictory evidence to Moscow's claims of military withdrawal.

The PM has accused Moscow of sending "mixed signals" over its build-up of troops near Ukraine.

Mr Johnson, who has spoken to a raft of world leaders about the Ukraine crisis in recent days, held a call with Japan's Prime Minister Fumio Kishida on Wednesday afternoon.

"Both agreed that the international community needed to stand united against an invasion of an independent country, and said they would not tolerate Russia's aggression," a Downing Street spokeswoman said.

"An invasion would shake the foundations of international order, and have severe consequences, they agreed."

US Secretary of State Antony Blinken said on Wednesday there had been no sign of "any pullback" of Russian forces.

"We continue to see critical units moving toward the border, not away from the border," he told MSNBC.

Putin keeping 'all the options open'

Ukraine's ambassador to the UK, Vadym Prystaiko, suggested that Russian President Vladimir Putin was currently keeping "all the options open".

"We are observing de-escalation, some de-escalation [but] still not enough to see actually they [Russia] are moving them [troops] back," he told Sky News.

"They have started moving around but at the same time they are bringing some other new fresh troops towards our border.

"So the message is still mixed and I believe that the decision hasn't been made by him [Mr Putin] yet and he has all the options open."