The Pentagon says five battleships and two aircraft carriers will leave the U.S. Naval Station in Norfolk, Virginia, and head to positions in the New York area and along the East Coast. The carriers are to provide upgraded air defense for the New York and Washington areas.
President Bush has left Barksdale Air Force Base in Louisiana aboard Air Force One and is flying to an undisclosed location.
He said earlier that all "appropriate" security measures are being taken, including putting the U.S. military on high alert worldwide.
Meanwhile the New York Subway has re-opened for school children and all commercial aircraft are now on the ground - the first time this has occurred in US aviation history.