The U.S. State Department updated its travel advisory for Ukraine on Friday.
"U.S. citizens in Ukraine should depart immediately if it is safe to do so using any commercial or other privately available ground transportation options," the advisory says.
The U.S. estimates thousands of Americans are still in Ukraine.
The State Department urges them to monitor the news about changing security conditions and alerts to shelter in place.
The new advisory comes as explosions have been reported in Ukraine's capital of Kyiv.
On Friday, Ukrainian Defense Ministry tweeted that Russian troops had entered the Obolon district, with reports of gunfire in central Kyiv.
"The security situation throughout Ukraine is highly volatile, and conditions may deteriorate without warning," the State Department said.
Ukraine's president said early Friday that more than 100 Ukrainians have been killed since Russia launched an invasion of the country.