President Joe Biden also warned that "everything could quickly become out of control" in the region.
"American citizens should leave now," Biden said in an interview filmed on February 10 with NBC News.
Referring to Russia, which is concentrating its troops near the Ukrainian border, President Biden said: "We are dealing with one of the largest troops in the world. It was a very different situation and things could quickly get out of control."
However, Mr. Biden affirmed that there was "no" situation that could force him to send the US military to pick up American citizens who wanted to leave Ukraine. "It's a world war when the Americans and the whole Russia start fire each other," Biden added.
The US leader said that Russian President Vladimir Putin "is just wise enough not to do anything that can negatively affect US citizens".