President Biden on Saturday warned Russian President Vladimir Putin that if Russia invaded Ukraine, the U.S. and its allies would "respond decisively and impose swift and severe costs on Russia," the White House said.
The two leaders spoke by phone for roughly an hour.
"President Biden reiterated that a further Russian invasion of Ukraine would produce widespread human suffering and diminish Russia's standing," the White House said in a statement.
While the U.S. has remained prepared to engage with Russia diplomatically, the White House said, "We are equally prepared for other scenarios."
A senior administration official described the call as "professional and substantive."
"There was no fundamental change in the dynamic that has been unfolding now for several weeks," the official said. "But we believe that we have put ideas on the table that would be in our and our allies interest to pursue that would enhance European security, and it would also address some of Russia's stated concerns."
The official added that it remains unclear whether Russia is interested in moving forward with its goals diplomatically rather than using force.
"Given the readily apparent steps Russia is taking on the ground in plain sight right before our eyes, the stakes of this are too high not to give Russia every chance to avoid an action that we believe would be catastrophic," the official said.
Ukrainian president urges against panic
Despite U.S. officials sounding the alarm, Ukrainian President Volodymyr Zelenskyy insists Ukrainian intelligence does not track with what the U.S. is saying and he does not believe an attack from Russia is imminent.
"I think there's too much out there about a full-scale war from Russia, and people are even naming dates. The best friend for our enemies is panic in our country, and all this information only creates panic, it doesn't help us," he said, during a visit to Kalanchak, a town near Russia-occupied Crimea.
State Department warns Americans to leave Ukraine immediately
For Americans on the ground in Ukraine, the U.S. State Department is warning that it is "past time" for them to leave, as the threat of a Russian invasion remains imminent. Late Friday night, the department directed U.S. Embassy staff in Kyiv to depart the country. The U.S. is also suspending most consular services in Ukraine starting Sunday.
The State Department's travel advisory for Ukraine has also been updated, telling Americans in that country to "depart immediately."
"We fervently hope and continue to work intensively to try to ensure that Ukraine does not become a war zone," a senior State Department official said on Saturday at a news briefing. "However, it appears increasingly likely that this is where this situation is headed, toward some kind of active conflict."
"That is why we are reducing our staff to a bare minimum while we still have the ability to get our official people out safely and in a predictable fashion," the official said.
While the U.S. Embassy is not closing, the staff remaining in Ukraine will conduct only essential business. Some staff is relocating to the office in Lviv in western Ukraine, as well, where emergency consular services will be available.
The State Department is not detailing the exact number of personnel staying in the country but says the staff remaining will include security support and communications staff.
The State Department official also said that while it is reducing its "official footprint" in Ukraine, the U.S. is still supporting the Ukrainian government and people. Ammunition from the U.S. is still arriving there, the official said.
The official said Ukrainian officials have remained "very professional" about the steps the U.S. is taking to decrease its presence, "even if all of them don't necessarily agree."
U.S. troops move out of Ukraine, reposition to other parts of Europe
Secretary of Defense Lloyd Austin has also ordered a temporary repositioning of U.S. troops that had been deployed to Ukraine since late November — another indication that the U.S. believes Russia's attack could come very soon.
Pentagon press secretary John Kirby said Saturday that 160 members of the Florida National Guard who had been training Ukrainian forces will move elsewhere in Europe.
"The Secretary made this decision out of an abundance of caution — with the safety and security of our personnel foremost in mind — and informed by the State Department's guidance on U.S. personnel in Ukraine," Kirby said in a statement.
"This repositioning does not signify a change in our determination to support Ukraine's Armed Forces, but will provide flexibility in assuring allies and deterring aggression," Kirby said.
To aid Americans who do decide to flee Ukraine, the U.S. is sending an additional 3,000 troops from the 82nd Airborne to Poland.
Blinken tells Russia a diplomatic path is still open
Biden's call with Putin was among several held between U.S. officials and their Russian counterparts.
Secretary of State Antony Blinken also spoke by phone with Russian Foreign Minister Sergey Lavrov on Saturday.
State Department spokesperson Ned Price said in a statement that Blinken made it clear that a "diplomatic path to resolving the crisis remained open, but it would require Moscow to deescalate and engage in good-faith discussions."
"[Blinken] reiterated that should Moscow pursue the path of aggression and further invade Ukraine, it would result in a resolute, massive, and united Transatlantic response," Price said.
Austin has also spoken with Russian Defense Minister Sergey Shoigu on Saturday, according to the Pentagon.
NPR's Franco Ordoñez and Frank Langfitt contributed to this report.