The United States said Thursday that Russia had not decided whether to invade Ukraine, but that the West was ready for all possibilities after a week of crisis talks.
Russian officials discussed tensions in a Vienna meeting with the Organization for Security and Co-operation in Europe (OSCE), the third round of diplomacy this week after talks directly with the United States and with NATO.
Jake Sullivan, President Joe Biden's national security advisor, said that the United States was under "no illusions" about Russia but hoped that a peaceful solution remained possible.
"We're ready either way," Sullivan told reporters at the White House.
"We're ready to make progress at the negotiating table," he said, "and we're ready to take the necessary and proper steps to defend our allies, support our partners and respond robustly to any naked aggression that might occur."
The West has accused Russia of deploying tanks, artillery and about 100,000 soldiers on Ukraine's war-torn eastern border in recent weeks, in what NATO says is preparation for an invasion.
But Moscow says this is a response to what it sees as the growing presence of NATO in its sphere of influence, where it fiercely opposes the expansion of the Atlantic alliance.
"The intelligence community has not made an assessment that the Russians have definitively decided to take a military course of action in Ukraine," Sullivan said.
"So as things stand right now, Russia has the opportunity to come to the table."
- 'Risk of war' -
The OSCE, set up during a Cold War detente phase as a forum that includes both the United States and Russia, took up the tensions in a 57-nation meeting in Vienna.
"It seems that the risk of war in the OSCE area is now greater than ever before in the last 30 years," said Foreign Minister Zbigniew Rau of Poland, which chairs the OSCE this year.
Helga Schmid, the OSCE secretary general, said the situation in the region was "perilous."
"It is imperative we find a way through diplomacy to deescalate and begin rebuilding trust, transparency and cooperation," she said.
Biden has warned Russia of major consequences if there is an invasion, with senators of his Democratic Party preparing a package that would include sanctions on President Vladimir Putin, his inner circle and banks as well as $500 million in fresh military aid to Ukraine.
Kremlin spokesman Dmitry Peskov said any personal sanctions against Putin would be "crossing a line and comparable to a rupture of ties."
- Russia 'disappointment' -
Russian Deputy Foreign Minister Sergei Ryabkov, who led talks with the United States, said in an interview with the Russian-language channel RTVI that he did not "see reasons to sit down in the coming days, to gather again and start the same discussions."
Russia's representative to the OSCE, Alexander Lukashevich, similarly said his country had expected more "substantial, in-depth discussions."
"To a very great extent it's a disappointment," he said, adding Putin would be briefed and "determine the level of readiness to take any further steps, in what format and in what timeframe."
Michael Carpenter, US ambassador to the OSCE, said the process of engagement was just beginning with the OSCE trying to further the diplomatic push.
"We never expected that there would be some kind of agreement today. Today is the announcement of the beginning of a process," he told reporters.
EU foreign policy chief Josep Borrell said Thursday that there should be no negotiations with Russian authorities over the fate of Ukraine so long as Moscow is massing troops at the country's border.
"Russian movements are part of the pressure," Borrell told journalists ahead of a meeting of EU ministers, insisting that there "should not be negotiation under pressure."
UN Secretary-General Antonio Guterres, however, said it was "absolutely essential that the dialogue that is taking place find a way allowing for de-escalation of tension."
The talks should lead to a way to "avoid any kind of confrontation that will be a disaster for Europe and for the world," he told reporters.
Talks are complicated by the unclear situation on the ground in rebel-held eastern Ukraine, where the OSCE has since 2014 been charged with ensuring peace accords are respected.
But that has failed to end fighting in the region, with conditions degrading for OSCE observers in areas controlled by pro-Russian separatists, a situation the US ambassador called "extremely worrying."