update(2:51pmET): The White House is now telling Americans in Ukraine to get out of the country "immediately" - giving a 24 to 48 hour time frame to make a safe exit. National Security Adviser Jake Sullivan urged all US citizens in Ukraine "to depart immediately." The number of Americans in the country is often estimated in the tens of thousands - likely somewhere around 30,000 - including dual citizens.

U.S. ORDERS ADDITIONAL 3,000 TROOPS TO POLAND

"We want to be crystal clear on this point. Any American in Ukraine should leave as soon as possible and in any event, in the next 24 to 48 hours," Sullivan said. "We obviously cannot predict the future. We don't know exactly what is going to happen, but the risk is now high enough and the threat is now immediate enough." If they stay, Americans "are assuming risk with no guarantee that there will be any other opportunity to leave and no prospect of a US Military evacuation in the event of a Russian invasion," he added.