Ukrainian soldiers during a military exercise in the Donetsk region in December 2021. (Photo: AP)

The order also allowed US diplomats stationed at the embassy in the Ukrainian capital Kyiv to leave voluntarily.

Officials "will not be in a position to evacuate American citizens in such a contingency, so US citizens currently present in Ukraine should plan accordingly," the US Embassy said in a statement.

The US State Department also advised US citizens not to travel to Russia nor to Ukraine, due to tensions along the Russia-Ukraine border.