WILMINGTON, Del. (AP) — Presidents Joe Biden and Vladimir Putin spoke frankly for nearly an hour late 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.

Biden reaffirmed the U.S. 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 U.S. move could lead to a complete rupture of ties between the nations.

“It 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.

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 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,

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.

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 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.

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 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.

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 U.S. 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 U.S. 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 U.S.-Russia relationship, are not expected to take part in the January talks.

Last week, Russia test-fired Zircon hypersonic missiles, a 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.

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

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.”

Last month, Putin voiced concern that NATO could potentially use the Ukrainian territory for the deployment of missiles that would be capable of reaching Moscow in just five minutes and said that Zircon would give Russia a comparable capability.

As Biden prepared for the talks with Putin, the administration also sought to highlight its commitment to Ukraine and drive home that Washington is committed to the “principle of nothing about you without you” in shaping policy that affects European allies. Secretary of State Antony Blinken spoke on Wednesday with Ukrainian President Volodymyr Zelenskyy.

Past military incursions by Putin loom large.

In 2014, Russian troops marched into the Black Sea peninsula of Crimea and seized the territory from Ukraine. Russia’s annexation of Crimea was one of the darker moments for President Barack Obama on the international stage.

The U.S.-Russia relationship was badly damaged near the end of President George W. Bush’s administration after Russia’s 2008 invasion of its neighbor Georgia after Georgian President Mikheil Saakashvili ordered his troops into the breakaway region of South Ossetia.

Biden, who is spending the week in his home state of Delaware, spoke to Putin from his home near Wilmington. The White House distributed a photo of the president speaking to the Russian leader from a desk lined with family photos.

Vladimir Isachenkov reported from Moscow. Associated Press writers Dasha Litvinova in Moscow and Yuras Karmanau in Kyiv, Ukraine, contributed to this report.