Insurgent bombers and gunmen killed four Iraqi national guardsmen, a police chief and two policemen Tuesday in the militants' unrelenting attacks against the country's security forces.

Also, two U.S. soldiers were killed and two others wounded by a roadside bomb late Monday in Iraq's capital, while two American Marines died of wounds received in fighting Monday in Anbar Province west of Baghdad (search), the military said. One of the Marines died of his wounds Tuesday.

A third Marine died Tuesday from a non-hostile gunshot wound, and another soldier died in a vehicle accident, the military said. The deaths brought to at least 918 the number of U.S. service members who have died since the beginning of military operations in Iraq.

The Iraqi national guardsmen were killed when a car bomb hit their post in the Kharnabat region just north of Baqouba. A white pickup truck sped toward the checkpoint and tried to merge with a convoy of U.S. military vehicles, said Maj. Neal O'Brien, a U.S. Army spokesman.

An Army driver forced the truck off the road and the attacker exploded the bomb, he said. Six guardsmen were also wounded, said national guard Lt. Mohammed al-Duleimi.

"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," O'Brien said.

The blast was part of a campaign of attacks against police and national guardsmen, whom insurgents view as collaborators with U.S. and coalition forces. From April 2003 to May 2004, 710 Iraqi police were killed out of a total force of 130,000 officers, authorities said. A week ago, a truck bomb targeted a police recruiting center in Baqouba (search), 35 miles northeast of Baghdad, where hundreds of job applicants were gathered. It killed 70 people.

Earlier Tuesday, a roadside bomb in Baghdad's al-Washash district killed Col. Mouyad Mohammed Bashar, chief of the al-Mamoun police station, and another officer. Associated Press Television News pictures showed a white police pickup truck, its doors smashed and blood splattered across the driver's seat.

A third officer was wounded, said Zayed Mohammed, a doctor at al-Yarmouk hospital. At the hospital, a bloodied policeman lay on a bed, bandages wrapped around his stomach and leg.

In the northern city of Mosul (search), attackers fired on a police station, killing one officer and wounding two before fleeing, said police chief Izzat Ibrahim.

In the holy city of Najaf, U.S. forces fought Monday with gunmen protecting radical Shiite cleric Muqtada al-Sadr's (search) house in clashes that killed one woman and wounded three people, according to the director of the Al-Hakim Hospital. The U.S. military had no immediate comment.

At least six U.S. military vehicles entered the Zahra area in Najaf near al-Sadr's house, which is protected by his militia, the Mahdi Army, witnesses said.

Barrages of gunfire and mortar rounds set cars on fire before Iraqi police intervened and the U.S. forces withdrew, witnesses said.

Ali al-Yassiry, a Baghdad spokesman for al-Sadr, said U.S. troops briefly surrounded al-Sadr's house but then withdrew from Najaf.

Al-Sadr, who is wanted by U.S. forces for the April 2003 murder of a moderate cleric in Najaf, was in his house at the time, witnesses said.

The radical cleric, who has grassroots support for his anti-coalition stance, began a two-month rebellion in early April after the U.S.-led occupation authority closed his newspaper and arrested a key aide. A series of truces ended the fighting, and the issue of whether to carry out the arrest warrant was postponed.

In northern Iraq, saboteurs bombed an oil pipeline northeast of the town or Beiji on Tuesday, the latest attack on the nation's infrastructure, the U.S. military said.

Police Col. Nurzad Ahmed, a security official at the state-run Northern Oil Company, said a fire was raging near al-Fattah, about 135 miles north of Baghdad, but the pipeline, which was not an export line, was not on fire. He said a nearby trash heap had been set ablaze.

"The fire is huge and we have started our efforts to put it out," Ahmed told The Associated Press.

Maj. Neal O'Brien, a spokesman for the U.S. Army's 1st Infantry Division, said the fire had spread to a nearby construction site.

It was unclear what effect, if any, the explosion would have on oil exports. Insurgents have repeatedly attacked Iraq's oil infrastructure in an effort deprive the interim government of money for reconstruction efforts.