REHOBOTH BEACH, Del. >> President Joe Biden and Vladimir Putin will speak Thursday as the Russian leader has stepped up his demands for security guarantees in Eastern Europe while maintaining an unsettling buildup of troops near Russia’s border with Ukraine.

The two leaders will discuss “a range of topics, including upcoming diplomatic engagements,” National Security Council spokeswoman Emily Horne said in a statement announcing the call.

The talks come as the U.S. and Western allies have watched the massing of Russian forces along the border, growing to an estimated 100,000 and fueling fears that Moscow is preparing to further invade Ukraine.

Secretary of State Antony Blinken spoke on Wednesday with Ukrainian President Volodymyr Zelenskyy.

State Department spokesman Ned Price said Blinken “reiterated the United States’ unwavering support for Ukraine’s independence, sovereignty, and territorial integrity in the face of Russia’s military buildup on Ukraine’s borders.”

Price said the two discussed efforts to peacefully resolve the conflict in eastern Ukraine and upcoming diplomatic engagements with Russia.

Putin said earlier this week he would ponder a slew of options if the West fails to meet his push for security guarantees precluding NATO’s expansion to Ukraine.

Earlier this month, Moscow submitted draft security documents demanding 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 next month to discuss its concerns.

The U.S. and Russia are to hold high-level talks on Jan. 10. Moscow and NATO representatives are expected to meet that same week as well as Russia and the Organization for Security and Co-operation in Europe, which includes the United States.

In Thursday’s call, which was requested by the Russians, Biden is expected to stress to Putin that the U.S. is united with its allies but will demonstrate a willingness to engage in “principled diplomacy” with Russia, according to a senior administration official who briefed reporters on the upcoming call. The official spoke on the condition of anonymity. The two leaders held a video call earlier this month.

The official added that the White House sees the leader- to-leader engagement as important as the administration looks to find a way beyond this “moment of crisis” over growing worries of a further Russian invasion of Ukraine.

In 2014, Russian troops marched into the Black Sea peninsula of Crimea and seized the territory from Ukraine. Russia’s annexation of Crimea — one of the darker moments for former President Barack Obama on the international stage — looms large as Biden looks to contain the current smoldering crisis.

White House national security adviser Jake Sullivan has made clear in public comments that the administration is ready to discuss Moscow’s concerns about NATO in talks with Russian officials, but emphasized that Washington is committed to the “principle of nothing about you without you” in shaping policy that affects European allies.

“We’re approaching the broader question of diplomacy with Russia from the point of view that … meaningful progress at the negotiating table, of course, will have to take place in a context of de-escalation rather than escalation,” Sullivan said at an event hosted by the Council on Foreign Relations earlier this month. He added “that it’s very difficult to see agreements getting consummated if we’re continuing to see an escalatory cycle.”

The two leaders are also expected during Thursday’s call to discuss efforts to persuade Iran to return to the 2015 nuclear accord, which was effectively scrapped by the Trump administration.

Despite differences on Ukraine and other issues, White House officials have said the Iran nuclear issue is one where they believe the U.S. and Russia can work cooperatively.

Kremlin spokesman Dmitry Peskov confirmed that Putin would speak with Biden on Thursday but provided no details.