White House national security adviser Jake Sullivan encouraged all Americans who remain in Ukraine "to depart immediately" as he detailed how the choreography of a possible Russian invasion of Ukraine could unfold.
"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," he added.
Sullivan said that if Americans stay they "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."
The national security adviser said that if Russia invades, then it would likely begin with an aerial bombing ahead of a ground invasion.
"If a Russian attack on Ukraine proceeds, it is likely to begin with aerial bombing and missile attacks that could obviously kill civilians without regard to their nationality. A subsequent ground invasion would involve the onslaught of a massive force. With virtually no notice, communications to arrange a departure could be severed and commercial transit halted," he added.
Sullivan also said: "I am not standing here and saying what is going to happen and not happen; I'm only standing here to say that the risk is now high enough and the threat is immediate enough that prudence demands that it is the time to leave now."