President Joe Biden and Russian President Vladimir Putin, seen here June 16 in Geneva, Switzerland, had a call concerning the recent buildup of Russian troops along the Ukrainian border this week.

WILMINGTON, Del. — Presidents Joe Biden and Vladimir Putin spoke Thursday amid growing alarm over Russia’s troop buildup near Ukraine, a simmering crisis that has become further complicated in recent days as the Kremlin has stepped up its calls for security guarantees and test fired hypersonic missiles to underscore its demands.

Putin requested the call, the second between the leaders this month, ahead of scheduled talks between senior U.S. and Russian officials set for Jan. 10 in Geneva.

White House officials said that the call began at 3:35 p.m. EST.

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 made clear are non-starters.

The White House said ahead of the call that Biden would tell 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 its demands for new guarantees from the U.S. and NATO.

Those 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 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, a career foreign service officer 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. 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.