Drive-by gunmen on Tuesday killed two Europeans working on a water project south of Baghdad (search), bringing to six the number of foreign aid workers killed in two days. The attacks apparently signal a strategy shift with insurgents taking aim at "soft" targets.

In an attack on Monday, four American missionaries were killed in a similar shooting in the northern city of Mosul. They had also been working on a water project.

The twin attacks are seen as a new effort by insurgents to snarl work by the U.S.-led coalition to rebuild Iraq in preparation for the American hand-over of authority to the Iraqis on June 30.

Also gunned down on Tuesday were three Iraqi police officers and a translator working for the U.S. military -- victims of a long-running rebel campaign to kill those perceived as collaborating with the United States.

At a military ceremony in Tikrit (search), Lt. Gen. Ricardo Sanchez (search), the top U.S. commander in Iraq, said the attacks on humanitarian workers were an attempt to intimidate those trying to help the 36-nation U.S.-led coalition.

"Clearly there has been a shift in the insurgency and the way the extremists are conducting operations," Sanchez said in an apparent reference to rebel assaults on civilians.

Two American employees of the coalition and an Iraqi translator died in a shooting on a road south of Baghdad on March 9. The Americans were the first civilians from the U.S. occupation authority to be killed in Iraq.

"It is very clear they are going after these targets that might create some splits within the coalition," Sanchez said.

The role of Spain in the coalition remained in doubt Tuesday after Prime Minister-elect Jose Luis Rodriguez Zapatero promised to withdraw the country's 1,300 troops by June 30 if the United Nations does not take control of peacekeeping. The Spanish troops operate south of Baghdad.

Honduras, which also was scheduled to end its mission at that time, said Tuesday it plans to bring home its 370 troops and would extend "only if the United Nations asks."

Zapatero's Socialist party was propelled to an upset victory in elections Sunday by anger over terrorist attacks in Madrid last week that killed 201 people. Voters accused Prime Minister Jose Maria Aznar of making Spain a target by supporting the U.S.-led war in Iraq (search).

On Tuesday, Sanchez said the coalition could manage without the Spaniards.

"It is something we will have to adjust to," Sanchez said. "But it is clearly manageable. It is not a significant military problem for the coalition to be able to cover that area."

The Tuesday killings involved a German and a Dutch national gunned down near the town of Mussayab (search), 45 miles south of Baghdad, officials said. Their Iraqi driver and a police officer also were killed in the attack. Two police officers were wounded.

Iraqi and U.S. officials earlier said the Europeans were Germans. But the Foreign Ministries of Germany and the Netherlands each confirmed one of their citizens had been killed. Names were not released.

Col. A'ayed Omran, police chief in Mussayab, said the two were water engineers working on a project at Al-Razzaza, a lake near the southern city of Karbala (search). He said they were carrying weapons because they had been attacked in the same area before.

The four U.S. missionaries slain in Mosul were working on a water-purification project. One of them died on the way to a U.S. military hospital in Baghdad early Tuesday, and a fifth was being treated.

In Mosul on Tuesday, assailants in a car fired on a police vehicle, killing three officers and wounding one, police said. The gunmen fled. In another shooting in Mosul, gunmen killed an Iraqi woman working as a translator for the U.S. military, the U.S.-led coalition said. Two of her family members were wounded in the attack on their vehicle.

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.

The Virginia-based Southern Baptist International Mission Board identified the four dead missionaries as Larry T. Elliott, 60, and Jean Dover Elliott, 58, of Cary, N.C.; Karen Denise Watson, 38, of Bakersfield, Calif.; and David E. McDonnall, 28, of Rowlett, Texas.

McDonnall died Tuesday morning on a helicopter taking him to a military hospital in Baghdad after four U.S. military surgeons worked for six hours to save his life, the mission board said.

McDonnall's wife, Carrie Taylor McDonnall, 26, of Rowlett, Texas, was in critical condition, the board said.

Lt. Col. Joseph Piek, a spokesman for American forces in Mosul, said the five Americans were traveling in one car on the eastern side of Mosul when they were attacked. Iraqi police and the FBI were investigating.

Christian missionaries in predominantly Muslim Iraq are viewed with suspicion by many residents who believe the foreigners are trying to convert them to their faith.

"They knew going into Iraq, they couldn't really share their Christian faith unless somebody asked them," said Larry Kingsley, a church deacon. "They were there in a humanitarian situation. They were people who just had a great heart for helping people out."

In the northern city of Kirkuk, three mortar shells on Tuesday hit the airport compound, where U.S. troops are based. Smoke billowed and sirens blared. There were no reported injuries.

There has been no official count of Iraqi casualties, either civilian or military, during or after the war. The U.S. government says it doesn't count such casualties. The Associated Press surveyed hospitals beginning March 20, 2003, and counted 3,240 civilian deaths.