Presidents Joe Biden and Vladimir Putin spoke frankly for nearly an hour early Friday amid growing alarm over Russia's troop buildup near Ukraine, a crisis that's deepened as the Kremlin has stiffened its insistence on border security guarantees and test fired hypersonic missiles to underscore its demands.

Putin's foreign affairs adviser said Biden reaffirmed the US threat of new sanctions against Russia in case of an escalation or invasion, to which Putin responded with a warning of his own that such a US move could lead to a complete rupture of ties.

"It would be a colossal mistake that would entail grave consequences,” said Yuri Ushakov. He added that Putin told Biden that Russia would act as the US would if offensive weapons were deployed near American borders.

White House officials said the leaders agreed there are areas where the two sides could make meaningful progress but also that there are differences that might be impossible to resolve.

White House press secretary Jen Psaki said that Biden “urged Russia to de-escalate tensions with Ukraine” and “made clear that the United States and its allies and partners will respond decisively if Russia further invades Ukraine".

Putin requested the call, the second between the leaders this month, ahead of scheduled talks between senior US and Russian officials January 9 and 10 in Geneva. The Geneva talks will be followed by a meeting of the Russia-NATO Council on January 12 and negotiations at the Organization for Security and Co-operation in Europe in Vienna on January 13,

White House officials said Thursday's call lasted 50 minutes, ending after midnight in Moscow.

Russia has made clear it wants a written commitment that Ukraine will never be allowed to join NATO and that the alliance's military equipment will not be positioned in former Soviet states, demands that the Biden administration has rejected.

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Biden told Putin that a diplomatic path remains open even as the Russians have moved an estimated 100,000 troops toward Ukraine and Kremlin officials have turned up the volume on their demands for new guarantees from the US and NATO.

White House officials said Biden made clear that the US stands ready to exact substantial economic pain through sanctions should Putin decide to take military action in Ukraine.

Ushakov said Putin reacted strongly.

Putin "noted that it would be a mistake that our ancestors would see as a grave error. A lot of mistakes have been made over the past 30 years, and we would better avoid more such mistakes in this situation,” Ushakov said.

Russia's demands are to be discussed during the talks in Geneva, but it remains unclear what, if anything, Biden would be willing to offer Putin in exchange for defusing the crisis.

Draft security documents Moscow submitted demand that NATO deny membership to Ukraine and other former Soviet countries and roll back its military deployments in Central and Eastern Europe.

The US and its allies have refused to offer Russia the kind of guarantees on Ukraine that Putin wants, citing NATO’s principle that membership is open to any qualifying country. They agreed, however, to hold talks with Russia to discuss its concerns.

The security proposal by Moscow has raised the question of whether Putin is making unrealistic demands in the expectation of a Western rejection that would give him a pretext to invade.

Steven Pifer, who served as US ambassador to Ukraine in the Clinton administration, said the Biden administration could engage on some elements of Russia's draft document if Moscow is serious about talks.

Meanwhile, key NATO members have made clear there is no appetite for expanding the alliance in the near future. The US and allies could also be receptive to language in the Russians' draft document calling for establishing new consultative mechanisms, such as the NATO-Russia Council and a hotline between NATO and Russia.

“The draft treaty’s proposed bar on any NATO military activity in Ukraine, eastern Europe, the Caucasus, or Central Asia is an overreach, but some measures to limit military exercises and activities on a reciprocal basis might be possible,” Pifer, who is now a senior fellow at Brookings Institution, wrote in an analysis for the Washington think tank.

Biden and Putin, who met in Geneva in June to discuss an array of tensions in the US-Russia relationship, are not expected to take part in the January talks.

Last week, Russia test-fired Zircon hypersonic missiles, a provocative move that Russian officials said was meant to help make Russia’s push for security guarantees “more convincing". The test was the first time Zircon missiles were launched in a salvo, indicating the completion of tests before the new missile enters service with the Russian navy next year and arms its cruisers, frigates and submarines.

US intelligence earlier this month determined that Russian planning was underway for a possible military offensive that could begin as soon as early 2022, but that Putin had yet to determine whether to move forward with it.

Oleksiy Danilov, the secretary of Ukraine’s Security and Defence Council, said Thursday his country believes there is no immediate threat of a major Russian invasion.

“Our experts say that the Russian Federation just physically can’t mount a big invasion of our territory,” Danilov said. “There is a time period needed for preparations.”

The US military has flown surveillance flights in Ukrainian airspace this week, including a flight on Friday by an Air Force E-8C JSTARS aircraft, according to Chuck Pritchard, a spokesman for US European Command. That plane is equipped to provide intelligence on ground forces.

Pritchard said such flights are conducted with European allies “routinely."

Russia has denied any intention of launching an invasion and, in turn, has accused Ukraine of hatching plans to try to reclaim control of territories held by Moscow-backed rebels by force. Ukraine has rejected the claim.

At the same time, Putin has warned that Moscow will have to take “adequate military-technical measures” if the West continues its “aggressive” course “on the threshold of our home".