U.S. Marines killed five insurgents and captured 10 others in a city west of Baghdad as American forces there stepped up their campaign to suppress deadly roadside bombs, the U.S. military said Tuesday.

Four Americans, meanwhile, died Monday evening in a suicide car bombing in southwestern Baghdad, the military said. A civilian translator was also killed in the attack.

According to a military statement, the five insurgents died Monday in Ramadi, 70 miles west of Baghdad, in a series of shootings that began when Marines discovered their attempts to plant bombs in a hole used by militants in the past to conceal explosives.

The incident occurred one day after Army snipers killed eight insurgents who were also trying to conceal explosives in Ramadi, capital of Iraq's most volatile province, Anbar.

Roadside bombs have become the major killer of American forces in Iraq, accounting for most of the 96 deaths among U.S. service members here last month.

On Tuesday, a senior member of the Iraqi police in Basra, Col. Mahmoud Qassim, was killed when a roadside bomb exploded near his convoy south of the city, police said. Another policeman also died in the attack.

A roadside bomb also killed a policeman and wounded three others Tuesday near the northern oil city of Kirkuk, police said.

The U.S. military released few details about the deaths of the four Americans and their translator in the suicide car bombings. They were members of the U.S. Army's Task Force Baghdad and were killed about 5 p.m. Monday, the military said.

Their deaths brought to at least 2,054 the number of U.S. military personnel who have died since the Iraq war started in 2003, according to an Associated Press count.

U.S. commanders have stepped up operations against the insurgents in hopes of establishing enough stability for national elections to go ahead as planned Dec. 15. U.S. officials hope to encourage a large turnout among Sunni Arabs to encourage many of them to lay down their arms and join the political process.

Sunni Arabs, who make up an estimated 20 percent of Iraq's 27 million people, form the core of the insurgency. Many of them boycotted the January election, enabling Shiites and Kurds to dominate the current parliament — a move that has led to further alienation among Sunnis.

In advance of the election, U.S. and Iraqi forces have launched a major offensive against the town of Husaybah, a major way station for foreign fighters entering the country from Syria.

Al Qaeda in Iraq apparently warned the Iraqi government Monday to halt the offensive within 24 hours or see "the earth ... shake beneath their feet."

"Let them know that the price will be very heavy," said an Internet statement purportedly issued by Al Qaeda, which has been blamed for some of Iraq's worst terror bombings. The warning's authenticity could not be confirmed.

Despite the threat, the chief of staff of Iraq's army, Gen. Babaker B. Shawkat Zebari, said U.S. and Iraqi forces will expand their operations in Husaybah to include other insurgent strongholds in the Euphrates River valley. He said operations were also planned in Diyala province northeast of Baghdad.

"Intelligence information indicates that terrorists are still coming from Syrian territories," Zebari told the Arabic newspaper Al-Hayat. "This is a very dangerous matter and this is what made us carry out series of attacks against areas on the Iraq-Syria border."

In a statement Monday on the Husaybah fighting, the Marines said American and Iraqi troops were trying to flush out insurgents in mosques, schools and other public buildings but did not say how much of the town had been secured.

The statement said at least 36 insurgents had been killed since the assault began Saturday in the town 200 miles northwest of Baghdad. A Marine commander gave the same figure Sunday night.

Also Monday, the U.S. military said five soldiers from the Army's 75th Ranger Regiment were charged with assault, maltreatment and dereliction of duty during a Sept. 7 incident "in which three detainees were allegedly punched and kicked while awaiting movement to a detention facility." All five were reassigned to administrative duties, the statement said.

The Army said the alleged incident occurred in Baghdad and that the detainees, all men, suffered bruises "caused by striking with a closed and open hand, kicking, and hitting with an object described as a broomstick."

Allegations of prisoner abuse at the Abu Ghraib prison in Baghdad gained international notoriety in 2004. Nine Army reservists were convicted in that scandal.

In other developments Tuesday:

• Police found six handcuffed corpses in a water treatment plant, police said.

• One civilian was killed when gunmen opened fire in the notorious Dora district in the south of the capital.

• A car bomb exploded near Mustansiriyah University, killing one person and injuring another.