WILMINGTON, Del. — President Biden assured Ukrainian President Volodymyr Zelensky Sunday that the United States “will respond decisively” if Russia invades the eastern European nation, according to the White House.

Days after a call with Russian President Putin, Biden spoke with Zelensky and reaffirmed previous commitments to aid Ukraine amid Russia’s military build up on Ukraine’s border that has prompted fears of an invasion, White House press secretary Jen Psaki said in a statement after the two leaders spoke by phone. The leaders “expressed support” for upcoming diplomatic talks, she said, that will commence in the wake of Putin telling Biden that any economic sanctions imposed in response to military action by the Kremlin could result in “a complete rupture of relations.”

“President Biden underscored the commitment of the United States and its allies and partners to the principle of ‘nothing about you without you,’” Psaki said after the conversation with Zelensky.

 

Zelensky said the call “proves the special nature of our relations.”

“Joint actions of [Ukraine, and the United States] and partners in keeping peace in Europe, preventing further escalation, reforms, deoligarchization were discussed,” he wrote in a tweet. “We appreciate the unwavering support of [Ukraine].”

Biden, who rang in the New Year at his Wilmington home, told reporters on Friday that he warned Putin during their 50-minute call Thursday that Russia would pay a “heavy price” if the country made further military moves against Ukraine.

Meanwhile, Putin told Biden that taking action against the nuclear superpower would be a mistake, “which our descendants will later appreciate as a huge one,” according to Russian presidential foreign policy aide Yuri Ushakov.

Biden’s calls with Putin and Zelensky mark an attempt to set the landscape ahead of diplomatic talks set to begin in Geneva on Jan. 9 and will the continue when the Russia-NATO Council meets on Jan. 12, followed by negotiations at the Organization for Security and Co-operation in Europe in Vienna on Jan. 13.

Security talks between U.S. and Russian officials come as Russia has assembled tens of thousands of troops along the Ukrainian border and fired a test salvo of hypersonic weapons.

The call with Zelensky is Biden’s latest attempt to quell tensions via telephone diplomacy amid mounting concerns of a military invasion. Zelensky spoke with U.S. Secretary of State Antony Blinken earlier this week.

While amassing troops at the border, Russia has issued an ultimatum to the United States and its European allies. Kremlin officials have demanded that Ukraine must be forever excluded from joining NATO and the U.S.-led military alliance must halt any other eastward expansion. But such terms have been rejected by Washington.

Looming over the negotiations is the area’s fraught history: In 2014, Russia annexed the Crimean Peninsula from Ukraine, as Moscow has backed separatists in the war-battered Donbas region.

How aggressively the United States should stand by Ukraine has also played a big role in U.S. domestic politics in recent years. House Democrats impeached President Trump in 2019 over allegations that he was withholding military and diplomatic support for Ukraine in an attempt to pressure Zelensky to investigate the role Biden’s son Hunter played with an Ukrainian energy company. At the time, Biden was considered the top Democratic presidential candidate.

The Senate acquitted Trump in early 2020, but some Republicans were critical of Trump’s failure to make the United States’ commitment to Ukraine clear even as they argued his actions were not an impeachable offense.