The US and Russia wrapped up their high-wire talks on the future of Eastern Europe on Monday, with the two rivals yet to find a breakthrough to calm the military stand-off in Ukraine.
Russia said any agreement would require progress on its security demands, with the two countries at odds over Nato's military presence in Moscow's orbit.
Sergei Ryabkov, Russia's Deputy Foreign Minister, said it was an “absolute imperative” for Moscow to block Nato expansion into Ukraine — a demand rejected by alliance members.
“We stressed very clearly that without any progress on these … absolutely necessary areas for us, then the whole issue would be under question,” Mr Ryabkov said.
But he said the situation was not hopeless after the all-day talks ended in Geneva, while the US said progress was possible if the Kremlin took “concrete steps” to de-escalate its troop build-up. It came as Ukraine sounded a grim warning of bodies piling up if diplomacy fails.
The talks began a week of diplomatic efforts that will include Nato and Russia meeting on Wednesday before a summit of the Organisation for Security and Co-operation in Europe on Thursday.
US Deputy Secretary of State Wendy Sherman said the talks were “frank and forthright” but that any breakthrough was likely to take weeks or longer.
She said Washington had offered more detailed talks on missile placement but had ruled out a ban on Ukrainian membership of Nato.
The US delegation was firm in “pushing back on security proposals that are simply non-starters”, she said.
“We will not allow anyone to close Nato's open-door policy.”
Ukrainian delegates separately met Nato officials in Brussels and said Russia had enough troops massed on its border to launch a full-scale invasion.
Olga Stefanishyna, a Ukrainian deputy prime minister, said any discussions on security arrangements “should start with the withdrawal of Russian troops from Ukrainian territory".
Russia annexed Crimea in 2014 and a pro-Kremlin insurgency has killed thousands of people in eastern Ukraine since then.
Amid fears that Russia is planning an invasion, Ms Stefanishyna said Kiev would keep channels of communications open with Moscow but that Russia's conditions for pulling back its tanks could not be considered a legitimate negotiating position.
These include its demand to stop any further expansion of Nato, an idea rejected by the alliance and the countries concerned, including Sweden, Finland and Ukraine.
“Nato allies are united in their support for all nations to choose their own path,” Secretary General Jens Stoltenberg said on Monday.
Mr Stoltenberg said the Russian military build-up was continuing with troops armed with “heavy capabilities”. He called on Moscow to de-escalate and be open about its military intentions.
He said he did not expect that this week's meetings could solve all the issues, but hoped that the parties could agree to further discussions and chart a way forward.
The UK Foreign, Commonwealth and Development office late on Monday said the UK was working closely with allies and partners, including the US, to make it clear to Russia that a campaign to subvert its democratic neighbours would not be accepted.
“Russia must stop its acts of aggression and pursue a path of diplomacy,” the office said.
“We are united with our allies and partners in condemning Russia’s threatening behaviour and we will hold Russia to its Helsinki, Budapest and Minsk commitments.
The UK also warned that “any military incursion into Ukraine would be a massive strategic mistake with severe costs.”
“At the Nato-Russia Council on Wednesday the UK, along with our allies, will tell Russia that its military build-up on Ukraine’s borders is utterly unacceptable,” the Foreign Office said.
Vadym Prystaiko, Ukraine’s ambassador to the UK, said he was concerned that the West was not prepared to push as hard as Russia to get its way over Ukraine.
Mr Prystaiko told Sky News that there would be “so many dead bodies returned to Moscow and Kiev” if Russian President Vladimir Putin orders troops into Ukraine.
“If he moves, we will be fighting,” Mr Prystaiko said. “Someday we have to draw the line".
He said he hoped Mr Putin's manoeuvres were mere sabre-rattling to push through Russia's demands for Eastern Europe.
At the US-Russia talks, the Moscow delegation said it was seeking legally-binding guarantees that Nato would not expand further east.
It said it would “not make any concessions while facing pressure and threats that are currently being levelled against Russia”, after Western powers have repeatedly warned of severe consequences if Ukraine is attacked.
Ms Sherman said US missile systems in Europe -- and not American troop deployments -- were discussed in the eight-hour-long dialogue.
“We also made clear that the United States is open to discussing the future of certain missile systems in Europe along the lines of the now defunct INF treaty (Intermediate-Range Nuclear Forces Treaty) between the US and Russia,” Ms Sherman, who led the US delegation, told reporters after the talks.
“We shared that we are also open to discussing ways we can set the cyclical limits on the size and scope of military exercises, and to improve transparency about those exercises, again, on a reciprocal basis,” she added.
This reciprocity is critical for the dialogue to succeed, she argued.
"Anything we do, that we might put on the table as an idea, we would expect reciprocal action from Russia. It may not be the exact same action, it may be another actually creates mutuality and reciprocity and that is very critical in our all arms control.”
She added that Russia needs to de-escalate tension with Ukraine and redeploy its troops amassed on its western border.
“They can prove that in fact, they have no intention [of invading Ukraine] by de-escalating and returning troops to barracks,” Ms Sherman said.
Monday’s talks took place in the Cold War trappings of Geneva, where a convoy of black vehicles with Russian diplomatic plates was ushered through the iron gates of the US diplomatic mission. Presidents Biden and Putin held talks in the Swiss city last June.