U.S. President Joe Biden was planning to talk Sunday with Ukraine’s President Volodymyr Zelenskiy to discuss ways to defuse tensions over Russia’s massive troop buildup along Ukraine’s eastern flank.

Biden has made little progress with Russian President Vladimir Putin to get him to withdraw about 100,000 troops stationed along Russia’s border with the former Soviet republic, although U.S. officials do not believe Putin has decided whether to invade Ukraine.

The U.S. and Russian leaders held a 50-minute phone call last Thursday, with Biden again warning Putin that the United States and its Western allies would impose significant economic sanctions against Moscow were Putin to carry out a Ukraine invasion, although Biden has ruled out a military response.

Moscow annexed Ukraine’s Crimean Peninsula in 2014, with the West protesting and imposing weaker sanctions.

The Kremlin in turn said last week that Putin told Biden in their call that new, tougher sanctions could lead to a complete rupture In Washington-Moscow relations.

The U.S. has been dispatching small arms and ammunition to Kyiv, along with Javelin missiles it says should only be used in defense.

The White House says that Russian and American officials will participate in three separate rounds of talks this month: first through bilateral talks scheduled to start January 10, and then through multiparty talks with the NATO-Russia Council, and with the Organization for Security and Cooperation in Europe.

“President Biden reiterated (in his call with Putin) that substantive progress in these dialogues can occur only in an environment of de-escalation rather than escalation,” White House press secretary Jen Psaki said.  

In the talks ahead, Russia is demanding that NATO, the seven-decade-old military alliance formed after World War II, deny membership to Ukraine and reduce its deployments in central and eastern Europe. White House officials have declined to discuss details of private talks. 

Some information for this report came from The Associated Press.