British Prime Minister Boris Johnson and US President Joe Biden agreed in a call this evening that a "crucial window for diplomacy" remained over the crisis in Ukraine, Mr Johnson's office said.
"They agreed there remained a crucial window for diplomacy and for Russia to step back from its threats towards Ukraine," a Downing Street spokesman added of the call between the transatlantic allies.
"The leaders emphasised that any further incursion into Ukraine would result in a protracted crisis for Russia, with far reaching damage for both Russia and the world."
The two leaders also stressed that diplomatic discussions with Russia remained "the first priority", and welcomed talks that have already taken place between Russia and NATO allies, according to the spokesman.
However the talks come as the US said it is relocating its embassy in Kyiv to the western Ukrainian city of Lviv, citing a "dramatic acceleration" in the build-up of Russian forces at the country's border.
"The embassy will remain engaged with the Ukrainian government," Secretary of State Antony Blinken said in a statement this evening, but "we strongly urge any remaining US citizens in Ukraine to leave the country immediately".
Russia has positioned more than 100,000 troops near to Ukraine's borders but denies planning to invade, accusing the West of hysteria.
In comments earlier, Mr Johnson warned an invasion of Ukraine could take place within 48 hours as he urged the Russian president to step back from the "edge of a precipice".
He said: "This is a very, very dangerous, difficult situation, we are on the edge of a precipice but there is still time for President Putin to step back."
Russia keeps door open for diplomacy
Meanwhile, Russia suggested that it was ready to keep talking to the West to try to defuse a security crisis in which it has massed a huge force within striking distance of Ukraine, while a Ukrainian official said Kyiv was prepared to make concessions to Moscow.
In a televised exchange, Russian President Vladimir Putin was shown asking his foreign minister, Sergei Lavrov, whether there was a chance of an agreement to address Russia's security concerns, or whether it was just being dragged into tortuous negotiations.
Mr Lavrov replied: "We have already warned more than once that we will not allow endless negotiations on questions that demand a solution today."
But he added: "Our possibilities are far from being exhausted, they certainly should not continue indefinitely, but I would suggest continuing and ramping them up."
Earlier, the Group of Seven large Western economies (G7) had warned Russia of "massive" economic consequences if it did invade and promised Kyiv swift support.
Ukraine's ambassador to Britain backtracked on remarks suggesting that Kyiv would reconsider its attempt to join NATO - one of Russia's primary concerns - but did say that other concessions could be on offer.
"We are not a member of NATO right now and to avoid war we are ready for many concessions and that is what we are doing in conversations with the Russians," he told the BBC in a clarification.
"It has nothing to do with NATO, which (membership application) is enshrined in the constitution."
The Kremlin said that if Ukraine renounced its aspiration to join the Western military alliance, it would significantly help address Russia's concerns.
Moscow has made clear it sees the former Soviet republic's quest for closer ties with the West, notably through NATO, as a threat.
Eight years ago, mass protests on Kyiv's Maidan square in favour of closer integration with the West forced out the pro-Russian president, Viktor Yanukovych.
Faced with the ascendancy of pro-Western politicians promising to advance democracy and fight corruption just across its border, Russia captured and then annexed Ukraine's Crimean peninsula, home to the Russian Black Sea fleet.
It also supported pro-Russian rebels who have seized part of Ukraine's industrial, largely Russian-speaking east in a war that is still adding to its toll of more than 14,000 lives lost.
The G7 finance ministers said fresh military aggression by Russia against Ukraine would trigger "economic and financial sanctions which will have massive and immediate consequences on the Russian economy".
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But talk of diplomatic efforts continuing brought the price of crude oil down off the seven-year highs it had hit earlier amid concerns that sanctions would disrupt exports from Russia, a major producer, in an already tight market.
Major European stock markets slumped by between 2.0% and 3.5% before making up some of their losses, as did Russian and Ukrainian bonds.
Sanctions could ultimately rebound on Western powers, which rely heavily on Russia for energy supplies, notably gas, as well as other raw materials.
European banks in particular fear that Russia could be excluded from the SWIFT global payment system, which would prevent the repayment of Russian debts.
The Dutch airline KLM has halted flights to Ukraine and through its airspace, while Germany's Lufthansa said it was considering a suspension, and British Airways flights appeared today to be avoiding Ukrainian airspace.
Ukraine International Airlines, the country's biggest carrier, said insurers had told it they would no longer cover its flights in Ukrainian airspace.
Mr Lavrov told Mr Putin the United States had put forward concrete proposals on reducing military risks, but that responses from NATO and the European Union - which has been at pains not to let Moscow divide its members - had not been satisfactory.
"None of my fellow ministers responded to my direct message," Mr Lavrov said. "Therefore we will continue to seek a concrete response from each country."
An EU official, who asked not to be named but has spoken to Mr Putin by phone in the past, said US talks with Russia were yielding little.
"Russia is trying to demonstrate that it is the policeman in the region," the source said. "The criticism by Moscow against Ukraine is this idea that the people made a choice for liberal democracy, values, principles and freedoms."
German Chancellor Olaf Scholz held talks in Kyiv with President Volodymyr Zelensky, to be followed tomorrow by a meeting with Mr Putin in Moscow.
He told reporters he saw "no reasonable justification" for Russia's military activity on Ukraine's border, and that Russia should accept offers to discuss European security. He announced a credit of €150m for Ukraine.
While Mr Zelensky affirmed that Ukraine still wanted to join NATO, Mr Scholz said it was strange that Russia had raised the issue now, when it was "not on the agenda".
Kyiv has long resented the German-Russian Nord Stream 2 project - a pipeline that will allow Russia to circumvent Ukraine in exporting gas to Germany - and has bristled at Germany's refusal to join other NATO partners in selling it weapons.
Germany did, however, begin sending troops today to help NATO member Lithuania bolster NATO's border with Russia.
Ukraine dubs Wednesday 'unity day' in face of feared invasion
Mr Zelensky has declared Wednesday - the day US officials warn might mark the start of a feared Russian invasion - national "unity day".
"They tell us that February 16 will be the day of the invasion. We will make this into Unity Day. I have signed the corresponding decree," Mr Zelensky said in a national address.
US intelligence reports briefed to international media suggest a strike could begin at some point after Mr Scholz concludes a round of talks with Mr Putin in the Kremlin tomorrow.