Four U.S. troops and seven members of the Iraqi security forces were killed late Monday and Tuesday in continuing violence across the country, including a car bombing near the central city of Baqubah and a roadside bomb in the capital.

Meanwhile, four Iraqis working for a French aid organization were reported Tuesday to have been stabbed to death near the city of Najaf, the scene of recurrent clashes between U.S. military forces and Shiite militia members loyal to the radical cleric Moqtada Sadr.

The latest attacks occurred as the interim prime minister, Ayad Allawi, returned home late Tuesday from a 10-day trip through the region aimed largely at seeking support from other Muslim countries to bolster security in Iraq by tightening borders and providing troops.

Also Tuesday, saboteurs bombed an oil pipeline in northern Iraq, causing a large fire to spread, and negotiations continued to free seven foreign truck drivers, taken hostage July 21 by Muslim militants who have demanded that foreign firms and troops withdraw from the country.

Four Iraqi National Guardsmen were killed and six were injured in the Baqubah incident Tuesday afternoon, U.S. and Iraqi officials said. A white pickup truck sped toward a guard post, attempting to reach a U.S. military convoy, and exploded as a driver in the convoy tried to force the truck off the road. No Americans were killed in the blast.

The car bomb was the second recent attack on security forces in Baqubah, where 70 people were killed last Wednesday when a minibus detonated in a crowded street outside a police station where hundreds of men were lining up to apply for jobs.

Meanwhile, in the affluent Baghdad district of Mansour, a roadside bomb killed an Iraqi police station chief, Col. Moayad Mahmoud Bashar, and a second officer. In the northern city of Mosul, gunmen fired at a police station, killing one officer and wounding two more before escaping.

"The continued targeting of Iraqi security force personnel undermines the security of all Iraqis and will only quicken the resolve of Iraqi security forces to provide a safe and secure environment," said Maj. Neal O'Brien, a U.S. military spokesman.

Officials said two U.S. soldiers were killed either late Monday or early Tuesday when a roadside bomb exploded in west Baghdad, but no further details were immediately available.

Military spokesmen also reported Tuesday that two U.S. Marines were killed in western Iraq overnight during clashes with insurgents. A total of 919 American troops have died since U.S.-led forces invaded Iraq 16 months ago.

In two other incidents, officials said one U.S. soldier died from "non-hostile fire," indicating an accidental shooting, and another was killed in a vehicle accident.

In the gruesome Najaf killings, officials of the Paris-based Agency for Technical Cooperation and Development, known as ACTED, said four of their Iraqi employees were apparently stabbed to death Friday after local militiamen discovered them near the site of a car bombing.

The group said the militiamen suspected the aid workers of being linked to the car bombing and summarily executed them in a scene of panicky violence just after the explosion. All the men were stabbed, and the eyes of one were gouged out. The men were reported missing Friday, and their bodies were found in the Najaf cemetery.

The city, a Shiite Muslim shrine center and the headquarters of Sadr and his youthful Mahdi Army militia, has been the scene of recurrent clashes. In the past few days, U.S. troops have surrounded Sadr's house and engaged in heavy armed clashes with his followers.