US President Joe Biden and his Russian counterpart Vladimir Putin on Thursday warned each other that an escalation of tension over Ukraine could rupture relations between the two countries, US and Russian officials said.
But the two leaders appeared set to move forward with further diplomatic engagement following their second call this month.
“President Biden urged Russia to de-escalate tension with Ukraine,” White House Press Secretary Jen Psaki said in a readout of the 50-minute call, which had been requested by Russia.
“He made clear that the United States and its allies and partners will respond decisively if Russia further invades Ukraine.”
The conversation marked the fourth time the two leaders have spoken this year and comes before a January 10 US-Russia security meeting, followed by a Russia-Nato session on January 12, and a broader conference including Moscow, Washington and other European countries scheduled for the following day.
“President Biden reiterated that substantive progress in these dialogues can occur only in an environment of de-escalation rather than escalation,” Ms Psaki said.
Mr Putin told Mr Biden that sanctions against Moscow could rupture ties between Russia and the US, the Kremlin said.
But Kremlin aide Yuri Ushakov told reporters Russia was satisfied with the conversation, which he said centred on security guarantees Moscow is seeking from the West amid a build-up of Russian forces on the Ukrainian border.
Mr Ushakov said Mr Biden appeared to agree that Moscow needed such guarantees and seemed serious about negotiations, even if differences remained.
A senior US official meanwhile told reporters the tone of the conversation was “serious and substantive".
“They each framed their positions as they've done in previous calls and also as they have done publicly,” the official said, noting that Mr Biden had outlined two tracks to Mr Putin on Ukraine.
“One is a path of diplomacy leading to de-escalation of the situation. The other path is more focused on deterrence, including serious costs and consequences.”
Moscow has alarmed the West by massing tens of thousands of troops near its border with Ukraine over the past two months following its seizure of Ukraine's Crimea peninsula in 2014 and its backing of separatists fighting in the eastern part of the country.
Russia denies planning to attack Ukraine and says it has the right to move its troops on its own soil as it likes.
The senior US official would not say whether the US believes Mr Putin has made the decision to invade.
On Wednesday, a different senior US official said Mr Biden will “make clear when he speaks with President Putin that we are prepared for diplomacy … but we are also prepared to respond if Russia advances with a further invasion of Ukraine”.
The official added that Washington is seeking a reduction in Russian troops on the border.
CNN reported that the US Air Force had flown another spy plane over eastern Ukraine on Thursday to gather intelligence on the Russian military situation. This was the second such flight carried out by the Pentagon this week.
Moscow, worried by what it says is the West's rearming of Ukraine, has said it wants legally binding guarantees Nato will not expand further eastward and that certain offensive weapons will not be sent to Ukraine or other neighbouring countries.
Other US officials said that despite a report at the weekend that Russia would be pulling back about 10,000 troops from its border with Ukraine, they had seen little evidence to support that so far.
“We are at a moment of crisis and have been for some weeks now given the Russian build-up, and it will take a high level of engagement to address this and to find a path of de-escalation,” said one of the US officials, who declined to be named.
US authorities have told Moscow they will take swift economic action against Russia and reinforce Nato in case of an invasion. But Mr Biden has been pushing direct diplomacy as an alternative.
Mr Putin has compared the current tension to the Cold War-era Cuban Missile Crisis in 1962. Washington regards some of his demands, including restrictions on Nato expansion, as non-starters.
Agencies contributed to this report