Russia said a “moment of truth” was looming in its stand-off with Nato as diplomats prepared on Tuesday for precarious talks to resume on the stand-off in Ukraine.
A day before a Nato-Russia summit, the alliance was huddling with US Deputy Secretary of State Wendy Sherman as she reported back on her talks with Moscow on Monday.
After those talks ended without agreement on calming tensions, Ms Sherman said the US was "working in lockstep" with its European allies to seek de-escalation from Moscow - addressing concerns from the EU and Ukraine over the former Cold War superpowers dominating discussions.
Ms Sherman and Nato Secretary-General Jens Stoltenberg "affirmed a unified Nato approach" in a meeting on Tuesday and agreed on the alliance's "unwavering support for Ukraine", the US delegate said.
Ms Sherman has signalled to Russia that progress will only be made if the Kremlin calls off its troop build-up near Ukraine.
But Russia is demanding security guarantees in exchange for any breakthrough – and said it would continue to press these demands at Wednesday’s talks.
Deputy Foreign Minister Alexander Grushko said Russia expected a “concrete, substantive, article-by-article reaction” to its proposals for a new treaty on European security.
Moscow’s terms include a veto on any further expansion of Nato, a demand rejected as a non-starter by the US, alliance members and the countries affected including Sweden, Finland and Ukraine.
“It's no exaggeration to say that a moment of truth is coming in our relations with the alliance,” Mr Grushko told Russian news agencies.
“We hope that this will be a serious, deep conversation on key, fundamental problems of European security.”
Kadri Liik, a Russia expert at the European Council on Foreign Relations, said Washington and Moscow appeared to have conceptually different views on what the talks should cover, with the US wanting to reduce them to technical arms control agreements while Russia wants to "redefine the whole European security order".
Nonetheless, the outcome of the talks - whether a wider agreement, a partial remedy or a descent into war - are likely to "shape Europe's strategic landscape for many years to come", she said.
Western diplomats, including Ms Sherman and Mr Stoltenberg, have warned Russia of severe consequences if it invades Ukraine, which Moscow insists it does not intend to do.
The US has promised to consult with its European allies in the talks. “Nothing about you without you. We will not make decisions about Ukraine without Ukraine,” Ms Sherman said on Tuesday.
After the Nato talks, the week’s meetings will continue on Thursday with a summit of the Organisation for Security and Co-operation in Europe. The US and Russia both signalled a willingness to talk further.
Kremlin spokesman Dmitry Peskov separately said while it was too early to assess the talks, Russia would not let them drag on indefinitely.
“There are no clear deadlines here, no one is setting them – there is just the Russian position that we will not be satisfied with the endless dragging out of this process," he said.
“There are not many reasons for optimism yet. It would be naive to believe that one round will give an exhaustive result.”
Ukraine’s President Volodymyr Zelenskiy meanwhile called for a further summit involving France, Germany and Russia to resolve the tensions.
Mr Zelenskiy held talks in Kiev with diplomatic advisers from Paris and Berlin, whose periodic four-way talks with Russia and Ukraine are known as the Normandy format.
“We are ready for the necessary decisions during the new summit of the leaders of the four countries,” said Mr Zelenskiy.
French President Emmanuel Macron signalled on Tuesday that he was open to more four-way talks, describing the Normandy format as playing a "structuring role" in negotiations.
He said it was "a very good thing" that the US and Russia were speaking face to face.