By ANNE GEARAN, AP Diplomatic Writer WASHINGTON – The United States reacted warily Monday to word that Syria will pull back its troops to the eastern part of Lebanon and demanded that it withdraw its forces “completely and immediately.” The announcement of a phased troop pullback was denounced by White House spokesman Scott McClellan as “a half measure.” The presidents of Syria and Lebanon announced Monday that the Syrian troops would be pulled back to eastern Lebanon’s Bekaa Valley by March 31. No timetable was announced for a full pullout.