Another 3,000 US troops are being sent to Poland.
The United States is also said to have plans to evacuate a large part of its embassy in Kiev - which, however, has not been confirmed by the State Department.
The tense situation around Ukraine in recent months has sharpened significantly since the United States warned that a Russian invasion could be imminent.
US National Security Adviser Jake Sullivan and Foreign Minister Antony Blinken base their warnings on intelligence that Russia now has enough military force on the Ukrainian border to carry out a major invasion.
But no more details have been published, which makes the situation difficult for outsiders to assess.
- It can start during the Olympics, said Sullivan.
Can invade during the Olympics, the United States claims
Ukraine, in turn, has warned that Russian-backed separatists in the eastern part of the country are conducting military exercises.
Kiev has otherwise tried to play down the threat from Russia, but now observers see the changed tone as another indication that the seriousness has intensified.
At a press conference on Friday, Jake Sullivan emphasized that the White House did not know whether Russian President Vladimir Putin had made a decision on an invasion - but warned that such a move could take place any day.
The attack could be limited in size or include an attack on Kiev, according to Sullivan.
Sullivan dismissed speculation that the Kremlin would never take the next step while the Olympics are taking place in Russia's close allies in China.
The Olympic Games will be held in the Beijing area until February 20.
Joe Biden in conversation with Vladimir Putin
Intensive diplomacy is underway to try to cool the situation.
US President Joe Biden will speak by telephone with Vladimir Putin on Saturday, the conversation is said to be scheduled for 17 Swedish time.
The French government has announced that President Emmanuel Macron will also call the Russian leader during the day.
Blinken will in turn talk to Russian Foreign Minister Sergei Lavrov