Biden's Sunday phone call with Ukraine's President Volodymyr Zelensky was the latest diplomatic effort to assuage mounting tensions following Russia's amassing of approximately 100,000 soldiers on Ukraine's eastern border. Washington, Moscow, and Nato member states are scheduled to meet in early January for talks, during which Russia will press for "security guarantees" aimed at containing the military alliance's expansion in Europe.

According to Jen Psaki, White House press secretary, Biden "reaffirmed" America's commitment to Ukraine's "sovereignty and territorial integrity."

Following the exchange, Zelensky tweeted that the leaders discussed "joint actions" by Ukraine, the US, and partners "in maintaining peace in Europe, preventing further escalation, reforming, and de-oligarchizing."

Biden's message to Zelensky echoed a phone conversation the US president had with Russia's President Vladimir Putin last week, during which the US president stated that Washington and its allies were prepared to respond "decisively" if Moscow invaded Ukraine.

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Although Russian President Vladimir Putin has previously denied any intention to invade Ukraine, he stated last month that he is prepared to use "appropriate military-technical measures" and "react harshly to hostile steps" if Kyiv and its western backers disregard Moscow's "red lines."

Russia's threat of military force has reignited debate in Finland over whether the Nordic country should join Nato, a move that would run counter to Russia's demands that the military alliance's expansion in Europe is limited.

Finland's president Sauli Niinisto and Prime Minister Sanna Marin both stated in their new year addresses that the country retained the option of applying for Nato membership at any time.

Additionally, the telephone calls come amid a debate over Russia's role in Europe's soaring gas prices. Certain European officials have accused Gazprom of withholding additional volumes as it prepares to launch the Nord Stream 2 pipeline to Europe, which is currently awaiting approval from German regulators.

Gazprom maintained that it was fulfilling all contractual obligations to supply gas to Europe and that record prices had stifled demand for spot sales.