President Joe Biden has warned Russia’s Vladimir Putin that the U.S. could impose new sanctions against Russia if it takes further military action against Ukraine, while Putin responded that such a U.S. move could lead to a complete rupture of ties between the nations.

The two leaders spoke frankly for nearly an hour Thursday amid growing alarm over Russia’s troop buildup near Ukraine, a crisis that has deepened as the Kremlin has stiffened its insistence on border security guarantees and test-fired hypersonic missiles to underscore its demands.

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Further U.S. sanctions “would be a colossal mistake that would entail grave consequences,” said Putin’s foreign affairs adviser Yuri Ushakov, who briefed reporters in Moscow after the Biden-Putin phone conversation. He added that Putin told Biden that Russia would act as the U.S. would if offensive weapons were deployed near American borders.

White House officials offered a far more muted post-call readout, suggesting the leaders agreed there are areas where the two sides can make meaningful progress but also differences that might be impossible to resolve.

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White House press secretary Jen Psaki said 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 U.S. and Russian officials Jan. 9 and 10 in Geneva. The Geneva talks will be followed by a meeting of the Russia-NATO Council on Jan. 12 and negotiations at the Organization for Security and Co-operation in Europe in Vienna on Jan. 13.

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

Biden told Putin the two powers now face “two paths”: diplomacy or American deterrence through sanctions, according to a senior administration official. Biden said the route taken, according to the official who briefed reporters on the condition of anonymity, will “depend on Russia’s actions in the period ahead.”

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 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 U.S. and NATO.

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

Putin reacted strongly.

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He “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.

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

The U.S. 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.