(FILES) This combination of pictures created on January 2, 2022 shows Russia s President Vladimir Putin at the Kremlin in Moscow on March 1, 2018, Ukrainian President Volodymyr Zelensky in Kiev on December 8, 2021 and US President Joe Biden in Wilmington, Delaware on January 15, 2021. (Photo by AFP)
With a Russian military buildup on Ukraine's borders, Biden "made clear" to Zelensky during a phone call that the "United States and its allies and partners will respond decisively if Russia further invades Ukraine," White House press secretary Jen Psaki said.
The show of US support for Ukraine comes days after Biden warned Russian President Vladimir Putin of severe consequences if Moscow launches a military invasion.
In his call with Zelensky, Biden also stressed Washington's commitment "to the principle of 'nothing about you without you,'" in an apparent reference to the need to include Ukraine in negotiations about its own future.
Zelensky tweeted afterwards that he appreciated the United States' "unwavering support" and that the call "proves the special nature" of the two countries' relationship.
High-ranking US and Russian officials are due to sit down on January 9 and 10 in Geneva to discuss the crisis.
Earlier this week, Biden spoke to Putin, the two leaders' second conversation by phone in three weeks as tensions mount over Ukraine.
Discussing the call with Putin on Friday, Biden said: "I'm not going to negotiate here in public, but we made it clear he cannot -- I'll emphasize, cannot -- invade Ukraine."
The US leader added, in remarks to reporters during a holiday stay in Delaware, that he had "made it clear to President Putin that we will have severe sanctions, we will increase our presence in Europe, with NATO allies" if Russia invades Ukraine.
Psaki also said in Sunday's post-call statement that Biden reaffirmed "the United States' commitment to Ukraine's sovereignty and territorial integrity."
The US leader additionally "expressed support for confidence-building measures to de-escalate tensions in Donbas and active diplomacy to advance the implementation of the Minsk Agreements."
Under the Minsk agreement -- brokered by France and Germany -- Ukraine agreed to carry out political reforms while Russia agreed to end its support for pro-Russian separatist rebels.
Washington and its European allies accuse Russia of threatening Ukraine with a new invasion.
Some 100,000 Russian troops are massed near the border of the country, where Putin already seized the Crimea region in 2014 and is accused of fomenting a pro-Russian separatist war which erupted that same year in the east.
Moscow describes the troop presence as protection against the expansion of NATO, although Ukraine has not been offered membership in the military alliance.
US Secretary of State Antony Blinken spoke Friday with NATO chief Jens Stoltenberg; afterwards, Blinken urged Russia to "engage meaningfully" in the upcoming talks on the tense standoff between Moscow and Kiev.
Stoltenberg said that NATO was "united" and "prepared for dialogue."
In Thursday's call, Biden warned Putin against invading Ukraine, while the Kremlin leader said anti-Moscow sanctions would be a "colossal mistake."
After a 50-minute phone call -- their second in just over three weeks -- both presidents indicated support for further diplomacy.
Putin was "pleased" overall with the talks, foreign policy adviser Yury Ushakov told reporters.
A senior US official, who spoke on condition of anonymity, said the tone "was serious and substantive."
But there was no disguising the depth of disagreement -- or the dangerously high stakes on the fringes of eastern Europe.