US Secretary of State Antony Blinken and Russian Foreign Minister Sergei Lavrov are to hold talks in Geneva later amid mounting fears that Russia could be about to invade Ukraine.
On Thursday Mr Blinken warned Moscow of grave consequences if any of its forces crossed the border.
Russia has 100,000 troops at the border, but denies planning to invade.
President Vladimir Putin has insisted that Ukraine should never be allowed to join Nato.
He also wants the defensive alliance to abandon military activity in eastern Europe.
Tensions over Ukraine
- EXPLAINED: Is Russia preparing to invade Ukraine?
- FROM KYIV: Ukrainians wait as Russia faces off with the West
- FROM BRUSSELS: EU ‘closest to war’ in decades over Russia-Ukraine crisis
- UK SUPPORT: UK says it is sending weapons to defend Ukraine
The summit between the top US and Russian diplomats follows moves by Mr Blinken to shore up support among US allies for sanctions against Mr Putin’s regime.
Following discussions in Berlin with British, French and German officials, Mr Blinken said on Thursday that allowing a Russian incursion into Ukraine would “drag us all back to a much more dangerous and unstable time, when this continent, and this city, were divided in two… with the threat of all-out war hanging over everyone’s heads”.
State Department officials have said that Mr Blinken will seek to offer Mr Lavrov a “diplomatic off-ramp” to ease tensions.
Speaking alongside Mr Blinken, Germany’s new Foreign Minister Annalena Baerbock pledged immediate action against any Russian invasion and did not rule out imposing measures that “could have economic consequences for ourselves”.
The UK’s Foreign Secretary Liz Truss has also called on Mr Putin to “desist and step back from Ukraine before he makes a massive strategic mistake” that would lead to terrible loss of life.
During a speech on Friday in Sydney, she urged Western powers to “step up” and warned that autocratic nations were being “emboldened in a way we haven’t seen since the cold war”.
Earlier this week, Britain announced it was supplying Ukraine with extra troops for training and defensive weapons.
The stakes for these talks are huge but there is little expectation they will produce a breakthrough to defuse the stand-off.
The Americans want to talk about avoiding war in Ukraine. The Russians want to talk about their demands for Nato to step back and allow Moscow to establish a new sphere of influence across Eastern Europe.
And all the while, the build-up of Russian troops and equipment continues, Ukrainian forces are holding their own exercises and some western powers are providing them with military assistance.
And yet US officials describe the talks as a good opportunity, to share concerns and find common ground. The Russians have agreed to come, having previously suggested diplomacy was at a dead end.
Antony Blinken and Sergei Lavrov both seem to agree they have much to discuss. And talking – for now – is not being accompanied by any fighting.
Mr Blinken’s comments come after US President Joe Biden on Wednesday predicted that Russia “will move in” on Ukraine and warned of a “disaster for Russia”.
But he also appeared to suggest that a “minor incursion” could attract a weaker response from the US and its allies.
The message provoked a rebuke from Ukrainian President Volodymyr Zelensky, who tweeted: “There are no minor incursions. Just as there are no minor casualties and little grief from the loss of loved ones.”
Mr Biden is also facing increasing calls from across the US political spectrum to take pre-emptive action against Russia.
On Wednesday, the senior Republican Senator Lindsey Graham called for “sanctions against Putin now,” while the Democratic Senator Richard Blumenthal has urged administration officials to begin “a massive airlift of … lethal weapons” to Ukrainian forces.
On Thursday the US also warned that Russian intelligence officers have been recruiting current and former Ukrainian government officials to step in as a provisional government and cooperate with an occupying Russian force in the event of an invasion.
The US Treasury Department imposed sanctions on two current Ukrainian members of parliament and two former government officials accused of being part of the plot.
“Russia has directed its intelligence services to recruit current and former Ukrainian government officials to prepare to take over the government of Ukraine and to control Ukraine’s critical infrastructure with an occupying Russian force,” a Treasury Department spokesperson said.