A clear sign Washington is bracing for aggressive moves by Russia.
On Monday, the State Department ordered the family members of government employees at the U.S. embassy in Ukraine to leave the country, and said it would allow diplomats stationed at the embassy to leave too if they wish.
Other Americans in Ukraine should also consider going, it said, using whatever commercial or private means they have, and it has warned against traveling to Ukraine or Russia.
For both countries, the State Department has issued its highest-level travel advisory, Level Four "Do Not Travel" citing what it says is an increased threat of significant Russian military action.
A State Department official said such action could come at any time, and that the U.S. government will not be in a position to evacuate Americans if it does.
Tensions in and near Ukraine have been increasing for months after Russia deployed some 100-thousand troops near Ukraine's borders.
Many in the West say Russia is preparing for war to prevent Ukraine from becoming a member of the NATO security alliance.
NATO's treaty states that if one member country is invaded or attacked by a third party, all members will collectively mobilize in its defense.
As for South Korea, the foreign ministry said it is closely monitoring the situation, and working to ensure citizens there are safe.
According to the ministry, there are about 800 Koreans living in Ukraine, none of whom live near the border with Russia.
The South Korean embassy in Kyiv, it said, is operating normally, and it noted that parts of Ukraine are already under the second-highest travel advisory, Level 3.
Bae Eun-ji, Arirang News.