chilling prediction about what a russian invasion would look like. >> if a russian attack on ukraine proceeds, it is likely to begin with a 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. >> reporter: sullivan warning, in no uncertain terms, american citizens in ukraine must leave now. >> any american in ukraine should leave as soon as possible, and in any event in the next 24 to 48 hours. if you stay, you are assuming risk with no guarantee that there will be any other opportunity to leave, and there no prospect of a u.s. military evacuation in the event of a russian invasion. >> reporter: the past 24 hours, filled with a frenzy of activity at the highest levels of the u.s. government. president biden convening a call with world leaders earlier today