According to a White House statement, US President Joe Biden assured his Ukrainian counterpart Volodymyr Zelensky on Sunday that Washington "will respond decisively" if Russia moves to invade its pro-Western neighbor.

With an alleged 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.

In his call with Zelensky, Biden also emphasized Washington's commitment "to the principle of 'nothing about you without you,'" an apparent reference to the need to include Ukraine in discussions about its own future.

After the call, Zelensky tweeted that he appreciated the US' "unwavering support" and that the call "proves the special nature" of the two countries' relationship.

I had a phone conversation with @SecBlinken. We agreed to continue consultations to reconcile positions and coordinate diplomatic efforts to achieve peace. I was assured of full 🇺🇸 support for 🇺🇦 in countering Russian aggression.

— Володимир Зеленський (@ZelenskyyUa) December 29, 2021

Russian-US meeting

Top US and Russian officials are scheduled to meet in Geneva on January 9 and 10 to discuss the crisis between the two nations.

Earlier this week, Biden spoke to Putin, in the two leaders' second conversation by phone in three weeks as tensions over Ukraine mount. 

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."

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In remarks to reporters during a holiday visit to Delaware, the US President added that if Russia invades Ukraine, he has "made it clear to President Putin that we will have severe sanctions, we will increase our presence in Europe, with NATO allies."

Psaki also said in Sunday's post-call statement that Biden reaffirmed "the United States' commitment to Ukraine’s sovereignty and territorial integrity."

Russia "very likely" to invade Ukraine

Russia is "very likely" to invade Ukraine and may be deterred only by "enormous sanctions", according to the Chairman of the US House Intelligence Committee on Sunday.

According to Adam Schiff, an invasion could backfire on Moscow by attracting more countries to the NATO military alliance.

“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 farther away,” he said.

On Twitter, Zelensky said, “The first international talk of the year with [Biden] proves the special nature of our relations.”

The first international talk of the year with @POTUS proves the special nature of our relations. Joint actions of 🇺🇦, 🇺🇸 and partners in keeping peace in Europe, preventing further escalation, reforms, deoligarchization were discussed. We appreciate the unwavering support of 🇺🇦.

— Володимир Зеленський (@ZelenskyyUa) January 2, 2022

He also said the joint actions of Ukraine, the US, “and partners in keeping peace in Europe, preventing further escalation, reforms, deoligarchization were discussed. We appreciate the unwavering support of Ukraine.”

Russia responds

Moreover, President Vladimir Putin had said last month that Moscow pursues a peaceful foreign policy; however, it has the right to defend its security.

Russia has been demanding a written commitment that Ukraine would never be able to join NATO and that the alliance would not place any military equipment in certain countries in the region surrounding Russia.

The Kremlin sees that it is best for Russian security that the alliance does not expand eastward and that Russia does not have any Western military activity in its vicinity.

Russia has repeatedly denied the Western allegations of intending to invade its western neighbor, and Moscow's Foreign Intelligence Service accused the US State Department of spreading false information about Russia's alleged troop buildup.