Biden assures Ukraine president US will respond 'swiftly' to Russian aggression

President Joe Biden assured Ukrainian President Volodymyr Zelensky that the United States and its allies will respond "swiftly and decisively" to any further aggression by Russia against Ukraine, according to a White House readout of a phone call between the two leaders.

The Sunday morning call took place as U.S. officials continue to warn that an attack from Russia could come "any day now" and urge all Americans still in Ukraine to leave the country.

"President Biden made clear that the United States would respond swiftly and decisively, together with its allies and partners, to any further Russian aggression against Ukraine," the White House said of the call. "The two leaders agreed on the importance of continuing to pursue diplomacy and deterrence in response to Russia's military build-up on Ukraine's borders."

In a tweet Sunday following the call, Zelensky said he and Biden discussed "security, economy, existing risks, sanctions and Russian aggression."

The conversation with the Ukrainian president comes one day after Biden spoke with Russian President Vladimir Putin, where Biden similarly warned that should Russia take action against Ukraine, the U.S., along with its allies, "will respond decisively and impose swift and severe costs on Russia."

According to a White House official, the call between Biden and Zelensky, who last spoke at the end of January, lasted 51 minutes, a shorter call than Biden's discussion with Putin on Saturday, which lasted just over an hour.

A senior administration official, speaking with reporters following the Saturday call with Putin, said the tone was "professional and substantive," however, "there was no fundamental change in the dynamic that has been unfolding now for several weeks."

Since Friday, the Biden administration has ramped up its warnings about a possible Russian attack on Ukraine -- which they say could happen as soon as this week.

"We have seen, over the course of the past 10 days, dramatic acceleration in the buildup of Russian forces and the disposition of those forces in such a way that they could launch a military action essentially at any time," National Security Adviser Jake Sullivan said on CBS on Sunday. "They could do so this coming week."

Despite the warnings, Zelensky has continued to call for calm in the country and seemed to express frustration with the dire tone.

"The best friend of our enemy is panic in our country, and all that information which helps create only panic doesn't help us," Zelensky said Saturday.