A mortar shell slammed into a coalition-run humanitarian aid office north of Baghdad (search), 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 (searchin 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 (search).

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.

In a shift of tactics earlier this week, gunmen fired into the mayor's office and courthouse in Fallujah and a police station in Khaldiyah (search) -- offices that have been working in coordination with Americans.

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.

"Abu Soheib, come back to us," wailed his wife Soheir, using his nickname. "Now there is no salary, and no man."

Iraqi security officials working with the Americans say regional leaders are directing the attacks by people still loyal to Saddam Hussein, former soldiers, Sunni Muslim radicals and non-Iraqi "holy warriors."

In Washington, Defense Secretary Donald Rumsfeld told reporters Wednesday at the Pentagon that groups of 10 to 20 people were behind the attacks on soldiers, not "large military formations."

Attacks and accidents have killed about 50 American troops -- including about a dozen from hostile fire -- since major combat was officially declared over on May 1. Between March 20, when the war started, and May 1, 138 Americans died from accidents or hostile fire.

The commander of the 4th Infantry Division, Maj. Gen. Ray Odierno, called the losses regrettable but "militarily insignificant."

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.

"One demonstrator pulled out a weapon and began shooting. U.S. forces responded, killing two of the demonstrators," said a military statement.

Samir Mizban, an Associated Press photographer, said a civilian driver fired a pistol into the air after crowds began smashing his car. Mizban said the protesters were stoning every vehicle within range.

"It was a new car. The demonstrators broke the windscreen with wooden sticks. The driver tried to escape, so he fired in the air with his pistol," said Mizban.

After the driver fired his gun, the enraged crowd threw rocks at the American soldiers blocking the palace gate and at journalists, who fled. That's when the U.S. troops opened fire, Mizban said.