Jake Sullivan, speaking at a White House briefing, says any American still in Ukraine should leave in the next 24-48 hours.
He said a Russian invasion could start with an air assault that would make departures difficult.
Four US officials told Reuters on Friday that Washington will send 3000 extra troops to Poland in coming days to try to help reassure NATO allies.
They are in addition to 8500 already on alert for deployment to Europe if needed.
Earlier, commercial satellite images from a US firm showed new Russian military deployments at several sites near the border.
The UK's Foreign Office updated its advice on Friday evening to urge UK nationals to "leave now while commercial means are still available".
Sullivan said US intelligence believes Russian President Vladimir Putin could order an invasion before the end of the Winter Olympics in Beijing on February 20 and that a rapid assault on the Ukraine capital Kyiv is a possibility.
Sullivan spoke on Friday after US President Joe Biden held a call on the crisis with the leaders of Britain, Canada, France, Germany, Poland and Romania, as well as the heads of NATO and the EU.
Following that meeting and with alarm spreading, British Prime Minister Boris Johnson joined a handful of other nations in urging their citizens to leave Ukraine.
Johnson told the meeting that he feared for the security of Europe and stressed the need for "a heavy package of economic sanctions ready to go, should Russia make the devastating and destructive decision to invade Ukraine", his office said.
Japan, Latvia, Norway and the Netherlands also told their citizens to leave Ukraine immediately, while Israel said it was evacuating relatives of embassy staff.
It remains unclear, Sullivan said, whether Putin has definitively given an order to start an invasion. He said he expects Biden to seek out a phone call with Putin soon on the crisis.
"We have not seen anything come to us that says a final decision has been taken, the go-order has been given," he said.
But with 100,000 troops massed on Ukraine's border, Sullivan said a Russian invasion could involve seizure of large sections of Ukraine as well as major cities including Kyiv.
Earlier, Russia's defence minister said Russia will respond "in the nearest future" to counterproposals on European security made by the United States and NATO.
Defence Minister Sergei Shoigu spoke with his UK counterpart Ben Wallace at a meeting in Moscow.
Russia has demanded that the US and the European Union guarantee the NATO military alliance will never let Ukraine join the bloc.
They have refused to do so but sent their own proposals to Russia.
In his starkest warning yet to US citizens in Ukraine to get out now, US President Joe Biden said he would not send troops to rescue US citizens in the event of a Russian assault.
"Things could go crazy quickly," Biden told NBC News.
US Secretary of State Antony Blinken also said an invasion could begin at any time.
"Simply put, we continue to see very troubling signs of Russian escalation, including new forces arriving at the Ukrainian border," Blinken said.
