NAJAF, Iraq (AP) -- U.S. warplanes bombed Najaf's Old City and troops clashed with Shiite militiamen Sunday amid fears a plan to end the standoff could collapse. An Iraqi mediator pleaded for radical cleric Muqtada al-Sadr to pull his fighters from the revered shrine at the center of the crisis. Meanwhile, four U.S. Marines assigned to the 1st Marine Expeditionary Force have been killed in separate incidents in Iraq's volatile Anbar province, the military announced. .... A car bomb also detonated near a U.S. military convoy outside Mosul, the military said. Two children were injured in the blast.