BAGHDAD, Iraq - U.S. and Iraqi military forces backed by helicopters and armored vehicles launched a big operation in Baghdad today, raiding suspected rebel hideouts in an effort to weed out insurgents and capture illegal weapons.

The campaign comes a day after gunmen killed two Europeans working on a water project. On Monday, assailants shot to death four American missionaries also working on a water project, suggesting the insurgents are going after civilians as a means of undermining reconstruction efforts in Iraq.

The new operation is called "Iron Promise" and is expected to involve thousands of U.S. troops from the Fort Hood, Texas-based 1st Cavalry Division, which has recently arrived in Iraq, and the outgoing Germany-based 1st Armored Division. Scores of Iraqi Civil Defense Corps soldiers are also involved.

In the first raid today, about 250 troops from the armored division's 1st Squadron, 1st Cavalry Regiment as well as 250 Iraqi soldiers fanned out across the sprawling 20th Street Market, in the city's Al-Bayaa district, which sells everything from vegetables to used car parts.

Some of the stores are suspected of supplying weapons to the rebels, said the raid's commander, Lt. Col. Chuck Williams, 40, from Sterling, Va. He said the market assault was just the start of a citywide crackdown on the guerrillas.

"There is a lot of pressure everywhere. It is all over town. The big things we are looking for are people moving weapons, IED (improvised explosive device) materials and explosives and ammunition. Our soldiers are looking to deter or discover this activity. We want to shut it off," he said.

With helicopters hovering overhead, forces in Bradley fighting vehicles and Humvees circled the market to prevent rebels from fleeing the area. Troops then went store to store searching for weapons and guerrilla suspects.

U.S. officials say they have identified 14 rebel cells across Baghdad and while raids on them in the past few months have disrupted their operations, they are still active.

A German and a Dutch national were killed in a drive-by shooting near the town of Mussayab, 45 miles south of Baghdad on Tuesday, officials said. Their Iraqi driver and a police officer also were killed, and two police were wounded.

Five Americans missionaries were shot in the northern city of Mosul a day earlier, leaving four dead and one wounded. The Virginia-based Southern Baptist International Mission Board identified the dead missionaries as Larry Elliott, 60, and Jean Dover Elliott, 58, of Cary, N.C.; Karen Denise Watson, 38, of Bakersfield, Calif.; and David McDonnall, 28, of Rowlett, Texas.

On Tuesday in Mosul, assailants in a car fired on a police vehicle, killing three officers and wounding a fourth, and separately gunmen killed an Iraqi woman working as a translator for the U.S. military.

Mosul was a prime recruiting ground for the officer corps of Saddam's army, and U.S. military officials have described the city as a hotbed of guerrilla activity. The CIA and other U.S. intelligence agencies have a unit in the city that is searching for so-called "high value" targets in northern Iraq.

Lt. Gen. Ricardo Sanchez, the top U.S. commander in Iraq, said such attacks were an attempt to divide the 36-nation coalition.