Russia signalled impatience on Thursday over a lack of progress in sensitive peace talks with the West, after diplomats were told the risk of war was at its highest in three decades.
Moscow's envoys said Russia would take "all necessary measures" - even hinting at military deployments to Cuba or Venezuela - to eliminate "unacceptable threats" if its demands to curb Nato expansion go nowhere.
Those demands were again rejected by the US at Thursday's summit of the Organisation for Security and Co-Operation in Europe, prolonging the stalemate in a week of high-wire diplomacy after US-Russia talks and a separate Nato-Russia meeting ended without a breakthrough.
“The drumbeat of war is sounding loud, and the rhetoric has gotten rather shrill", said Michael Carpenter, the US envoy to the OSCE, with diplomats on guard for a potential Russian invasion of Ukraine.
"We have to prepare for the eventuality that there could be an escalation – and that is why President [Joe] Biden said that the United States, together with our allies and partners – are prepared to impose massive costs", Mr Carpenter said, alluding to the potential economic sanctions that Washington has been coordinating with its European partners.
The Vienna summit of the 57-member OSCE brought together Nato members and Russia, as well as countries such as Ukraine, Sweden and Finland, which are not part of the US-led alliance.
Russia is seeking a veto on any of these countries joining Nato and limits on the alliance stationing troops in Eastern Europe. The US describes these demands as non-starters and says Russia must pull back its troops from Ukraine's eastern border for any progress to be made.
“For several weeks, we have been faced with the possibility of a major military escalation in Eastern Europe,” said Zbigniew Rau, the Polish foreign minister and chairman of the OSCE. “It seems that the risk of war in the OSCE area is now greater than ever before in the last 30 years."
Moscow's delegate to the talks, Alexander Lukashevich, said Russia's proposals were "not a menu from which to choose convenient items", after the US indicated it was willing to discuss arms control but not a limit on Nato expansion.
He said attempts to "drag out" negotiations would lead to an "inevitable deterioration of the security situation".
"If we do not hear a constructive response to the proposals made within a reasonable time frame, and the aggressive line of behaviour towards Russia continues, we will be forced to draw appropriate conclusions and take all necessary measures", he said.
Speaking separately, Deputy Foreign Minister Sergei Ryabkov told Russian television that he could neither confirm nor exclude the possibility of sending military assets to Cuba or Venezuela.
A refusal by the US and its allies to consider key Russian demands would raise doubts about continuing the talks, he said.
Western powers said it was the Kremlin raising tensions with a build-up of 100,000 troops near Ukraine, although Moscow denies planning to invade. Russia annexed Crimea in 2014 and pro-Kremlin separatists have waged a years-long war in eastern Ukraine.
Mr Carpenter said the US would not back down on keeping Nato’s door open to further members or stationing troops in Moscow’s former sphere of influence.
But he sought to reassure Russia that Nato’s military presence in countries such as Latvia, Lithuania and Estonia was “minimal and defensive in nature”.
Neil Bush, the UK's ambassador to the OSCE, called on Russia to de-escalate, remove its troops from Crimea and respect its international commitments.
"We are determined to support Ukraine’s sovereignty and territorial integrity in the face of Russia’s threatening behaviour," he said.
EU foreign policy chief Josep Borrell said there should be no agreement with Russia over the fate of Ukraine so long as Moscow is massing troops at the country's border.
After previously expressing concern about the US and Russia dominating talks, he said at an EU defence and foreign policy meeting that he was content with Washington's co-operation with Brussels.
"We have developed an extremely advanced co-ordination with the United States," he said. "We are in close co-operation and we are assured that nothing will be decided, nor even negotiated with the Russians without a close co-ordination with Europe and without the participation of the Europeans.”
Mr Rau, who took on the rotating OSCE chairmanship on January 1, told diplomats they should focus on a peaceful resolution of the Ukraine conflict.
“The problem is not related to one or two countries but poses a challenge to the stability and security of a European system that has been developed over the course of more than three decades," he said.
The OSCE was not indifferent to security concerns expressed by members and should be open to dialogue, said Mr Rau.
But he said any resolution of the Ukraine conflict should respect the sovereignty and territorial integrity of the country “within its internationally recognised borders”, which include Crimea.
The OSCE has run a monitoring mission in Ukraine since 2014. Its observers said on Wednesday that there had been hundreds of recent breaches of the ceasefire and that the mission’s freedom of movement was restricted.