President Joe Biden met with Ukraine's leader on Sunday to discuss Russia's army buildup along the country's border, warning that the US and its allies will act "decisively" if Russia invades Ukraine again.

Mr. Biden and President Volodymyr Zelensky's appeal came as the US and Western allies prepared for a series of diplomatic meetings to de-escalate a situation that Moscow has warned may jeopardize relations with Washington.

"President Biden made clear that the United States and its allies and partners will respond decisively if Russia further invades Ukraine," White House press secretary Jen Psaki said following the call.

Ms. Psaki continued, Mr. Biden reaffirmed his adherence to the idea of "nothing about you without you," the tenet that the US will not negotiate policies affecting Europe without the consent of its partners.

Mr. Biden has warned of imposing crippling penalties on Russia if it goes into Ukraine's territory, but he stated last month that US military intervention is not an option.

The Kremlin has demanded that any additional Nato enlargement exclude Ukraine and other former Soviet republics. Additionally, the Russians have asked that the military alliance withdraw offensive equipment from the region's governments.

The White House has rebuffed Russia's demands on Nato as non-starters. A central tenet of the Nato alliance is that membership is accessible to any country that meets specific criteria. And no outsider has veto authority over membership. While it is unlikely that Ukraine would join the alliance anytime soon, the US and its partners will not rule it out.

Mr. Zelensky stated on Twitter following Sunday's call that "keeping peace in Europe, preventing further escalation, reforms, deoligarchisation were discussed."

"We appreciate the unwavering support," Mr. Zelensky stated.

The US has made little progress in persuading Russian President Vladimir Putin to de-escalate hostilities. On January 9-10, senior US and Russian officials are expected to meet in Geneva to examine the issue. Following those discussions, the Nato-Russia Council and the Organization for Security and Cooperation in Europe will meet.

Mr. Biden spoke for nearly an hour with Mr. Putin on Thursday. He warned Mr. Putin the following day that his economy would suffer a "heavy price" if Russia, which has massed some 100,000 troops near the border, made further moves against Ukraine.

"I'm not going to negotiate here in public, but we made it clear that he cannot - I emphasise cannot - move on Ukraine," Mr. Biden said on Friday.

Mr. Biden stated that he told Mr. Putin that the Russians needed to resolve the problem before those meetings. In recounting the presidents' talk this week, Mr. Putin's foreign affairs adviser stated that Mr. Biden's pursuit of sanctions "could lead to a complete rupture of relations between our countries and Russia-West relations will be severely damaged."

The US President maintains his optimism.

According to US intelligence reports, Russia has been preparing for a possible invasion in early 2022. However, White House officials maintain that it is uncertain whether Mr. Putin has already decided to proceed with military strikes.

Mr. Biden, however, expressed optimism for the next meetings. Officials at the White House have stated that they will consult closely with Western allies.

"I always expect if you negotiate you make progress, but we'll see," he remarked on Friday. "We'll see."

According to US reports, Russian troops are massing near the Ukraine border.

Mr. Putin's previous military incursions loom heavily as Mr. Biden considers his subsequent actions.

Russian troops marched into Ukraine's Black Sea peninsula of Crimea in 2014 and seized the territory. Russia's invasion of Crimea was one of President Barack Obama's darkest international moments.

Near the end of President George W. Bush's term, the US-Russia relationship was severely damaged by Russia's 2008 invasion of neighboring Georgia after Georgian President Mikheil Saakashvili ordered his soldiers into the breakaway province of South Ossetia.

Rep. Adam Schiff, chairman of the House Intelligence Committee, said on Sunday that he felt Mr. Putin was hell-bent on invading Ukraine and that "nothing other than a level of sanctions that Russia has never seen will deter him" would stop him.

"Russia needs to understand we are united in this," Mr. Schiff told CBS's Face the Nation. "I also think that a powerful deterrent is the understanding that if they do invade, it is going to bring (Nato) closer to Russia, not push it further away."