NAJAF, Iraq, Aug. 9 - American forces besieging militiamen of a rebel cleric in a shrine and cemetery sacred to Shiite Muslims tightened their cordon on Monday, United States military officials in Baghdad said, and they warned the rebels that they could not receive any outside support. But the warnings drew an immediate riposte from the cleric, Moktada al-Sadr, who emerged from days of silence to reject demands for the militiamen to surrender. "I will defend Najaf until the last drop of my blood," Mr. Sadr said at a news conference in the Imam Ali shrine, which has served as...