to invade positioned along ukraine's borders. >> any american in ukraine should leave as soon as possible, and in any event, in the next 24-48 hours. >> reporter: about 7,000 americans in ukraine have registered with the state department. national security adviser jake sullivan said yesterday, anyone who stays should not expect a u.s. military rescue. >> the president will not be putting the lives of our men and women in uniform at risk by sending them into a war zone. >> reporter: 3,000 u.s. troops, however, are headed to join units already positioned in poland and romania, with 8,200 on standby to back up other nato allies, in case putin's territorial aspirations expand beyond ukraine. >> jiang: and christina joins us live from the white house now. christina, jake sullivan said it was an urgent message for those american and state department officials to get out of ukraine. what can you tell us about how they're leaving and where they are going? >> reporter: well, weijia, planes, trains, automobiles--