At least three U.S. soldiers were killed in sothern part of Baghdad, AL-Jazeera satellite channel reported, without giving further details. Only hours before attack, a mortar shell slammed into a coalition-run humanitarian aid office north of Baghdad, killing an Iraqi and wounding 12 others, the latest of several attacks targeting Iraqis working with U.S. forces, the military said Thursday. No U.S. forces were hurt in the attack. The mortar round hit the Civil Military Operations Center in Samarra, 75 miles north of Baghdad on Wednesday. U.S. soldiers heard three explosions and asked local police to investigate, said a statement from U.S. Central Command. The statement said Samarra police found the injured and killed and that soldiers were unable to find the attackers. The office coordinates between the military and civilian, nongovernment agencies in humanitarian assistance. Iraqi cities have been on edge since Sunday, when coalition forces began house-to-house searches in Baghdad for banned weapons and suspected activists trying to undermine the U.S.-led occupation. Iraqis also mourned two men shot by U.S. forces during a demonstration of disgruntled former army officers - the same day an American soldier was shot dead at a propane distribution point. In the Horreya district of Baghdad, mourners fired assault rifles in the air as they brought home the body of Taraq Hussein Mohammed, a 32-year-old former noncommissioned officer, one of two men shot outside the gate of the U.S. administration headquarters during a demonstration to protest the lack of jobs. An American soldier was killed Wednesday by two gunmen who walked to the propane distribution point that his squad was guarding and shot him at close range. A second soldier was wounded pursuing the gunmen, who escaped in a waiting car. He was the second soldier to die in Baghdad this week. On Monday, a sniper killed a soldier patrolling the dark streets in his vehicle with a single shot in the back. Demonstrations have taken place regularly outside the arched gate leading to the compound of Saddam's former Republican Palace. Past protests, usually by former soldiers and civil servants demanding new jobs or back wages, have been largely peaceful. The U.S. Central Command said the violence during Wednesday's demonstration began when protesters tossed rocks at a U.S. convoy it tried to pass through, but Iraqis said it's not true. **PHOTO CAPTION*** An wounded man is carried past a checkpoint of the 1st Armored Division near the area where a U.S. soldier was earlier shot dead and another wounded by two gunmen in Baghdad, Wednesday, June 18, 2003. (AP Photo/Denis Doyle)