President Joe Biden on Saturday again called on President Vladimir Putin to pull back more than 100,000 Russian troops massed near Ukraine's borders.
He also warned that the US and its allies would "respond decisively and impose swift and severe costs" if Russia invades, according to the White House.
Mr Biden and Mr Putin spoke for more than an hour the day after the US president's national security adviser, Jake Sullivan, warned that US intelligence shows that a Russian invasion could begin within days and before the Winter Olympics in Beijing end 20 February.
The Biden administration has been warning for weeks that Russia could invade Ukraine soon, but US officials had previously said the Kremlin would likely wait until after the Games ended so as not to antagonise China.
Mr Sullivan told reporters on Friday that US intelligence gleaned show that Russia could take military action during the Olympics.
Russia denies that it intends to launch an offensive against Ukraine.
Before talking to Mr Biden, Mr Putin had a telephone call with French President Emmanuel Macron, who met with him in Moscow earlier in the week to try to resolve the biggest security crisis between Russia and the West since the Cold War.
A Kremlin summary of the call suggested that little progress was made toward cooling down the tensions.
In a sign that American officials were getting ready for a worst-case scenario, the United States announced plans to evacuate its embassy in the Ukrainian capital, and Britain joined other European nations in urging its citizens to leave Ukraine.
The timing of any possible Russian military action remained a key question.
The US picked up intelligence that Russia is looking at Wednesday as a target date, according to a US official familiar with the findings.
The official, who was not authorised to speak publicly and did so only on condition of anonymity, would not say how definitive the intelligence was.
The White House publicly underscored that the US does not know with certainty whether Mr Putin is committed to invasion.
However, US officials said anew that Russia's build-up of firepower near Ukraine has reached the point where it could invade on short notice.
A Kremlin statement about the Putin-Macron call referred to "provocative speculations about an allegedly planned Russian 'invasion' of Ukraine."
Mr Putin complained in the call that the US and NATO have not responded satisfactorily to Russian demands that Ukraine be prohibited from joining the military alliance and that NATO pull back forces from Eastern Europe.
Mr Biden has said the US military will not enter a war in Ukraine, but he has promised severe economic sanctions against Moscow, in concert with international allies.
US Secretary of State Antony Blinken said he told his Russian counterpart Saturday that "further Russian aggression would be met with a resolute, massive and united trans-Atlantic response."
Meanwhile, Ukrainian President Volodymyr Zelenskyy tried to project calm as he observed military exercises Saturday near Crimea, the peninsula that Russia annexed from Ukraine in 2014.
"We are not afraid, we're without panic, all is under control," he said.
