- President Joe Biden and Russian President Vladimir Putin spoke in what is the second known call between the leaders this month.
- The phone call comes as tensions rise over a significant Russian military buildup on the Ukrainian border.
- For months, Ukraine has warned the United States and European allies that thousands of Russian troops were massing along its eastern border.
- The buildup has evoked shades of Russia's 2014 annexation of Crimea, a peninsula on the Black Sea, which sparked an international uproar and triggered a series of sanctions on Moscow.
The call began at 3:35 p.m. ET and lasted 50 minutes, the White House said.
White House press secretary Jen Psaki said Biden urged Putin to de-escalate tensions with Ukraine and that his administration was prepared to "respond decisively" alongside allies and partners if Russia further invades Ukraine.
A senior administration official, who declined to be named in order to discuss details of the call, said that the tone of the conversation was "serious and substantive."
"Both leaders acknowledged that there were likely to be areas where we could make meaningful progress as well as areas where agreements may be impossible," the official said, adding that the upcoming Jan. 10 security talks will build upon Biden and Putin's Thursday discussion.
Ahead of those talks, the senior administration official said that the United States and European allies will continue to watch developments in Ukraine.
"We're going to continue to monitor very closely the movement and build-up of Russian forces on the Ukraine border and prepare ourselves for whatever decision ultimately is made by the Russian president," the official said.
A Putin aide told reporters in Moscow that both leaders congratulated each other on the holidays and mutually wished success to the Russian and American people.
The aide described the conversation between Putin and Biden as "frank, informative and specific in nature."
"Putin outlined in detail the basic principles laid down by Russia in the security proposals and emphasized that we will seek to ensure Russia's security. In principle, the U.S. president agreed with this point of view," explained Yury Ushakov, according to an NBC News translation.
"Biden clearly stated that the United States does not intend to deploy offensive strike weapons in Ukraine. And Putin noted that this is one of the key points for us," Ushakov added.
The call, the second known discussion between the two leaders this month, was scheduled at Putin's request. The Russian president has previously insisted that despite a massive deployment of thousands of troops along Ukraine's border, Moscow is not preparing for an invasion of its ex-Soviet neighbor.
But Putin has laid out conditions for nonaggression: He has promised that Russian troops will not attack Ukraine if Kyiv's ongoing bid to join NATO is denied. Russia has described NATO's eastward expansion as a "red line" that poses security threats to Moscow.
Since 2002, Ukraine has sought entry into the world's most powerful military alliance, where the group's Article 5 clause states that an attack on one member country is considered an attack on all of them.
During a prior call earlier this month, Biden did not accept Putin's "red line" and instead warned that Washington and European allies were prepared to impose a web of economic and political countermeasures if Ukraine's sovereign borders were breached.
"We are prepared for diplomacy and for a diplomatic path forward, but we are also prepared to respond if Russia advances with a further invasion of Ukraine," a senior Biden administration official, who spoke on the condition of anonymity in order to share details ahead of the call, said Wednesday.
"We have coordinated with our allies to impose severe sanctions on the Russian economy and financial system far beyond what was implemented in 2014," the official said, referring to Moscow's 2014 invasion of Crimea.
For months, Ukraine has warned the United States and European allies that thousands of Russian troops were massing along its eastern border. The buildup has evoked shades of Russia's 2014 annexation of Crimea, a peninsula on the Black Sea, which sparked an international uproar and triggered a series of sanctions on Moscow.
Simon Miles, an assistant professor at Duke University's Sanford School of Public Policy, described Biden and Putin's discussions as productive but that a solution would need to involve the Ukrainian government.
"Today's phone call between presidents Biden and Putin comes at a critical point in European security. Russian troops are on Ukraine's border in significant numbers, and in a configuration which has analysts rightly worried about offensive military action," wrote Miles, an expert on Russia and the former Soviet Union.
"But one thing is clear: This is a crisis of the Kremlin's making," Miles said, adding, "What Putin's endgame is remains unclear."
Earlier this month, Ukraine's foreign minister told CNBC that Russia was in a position to quickly invade if Putin decided to carry out such an operation.
"Putin has not decided yet whether to do a military operation," Dmytro Kuleba told CNBC. "But if he decides to do so, things will happen in the blink of an eye."