Shiite Muslim militia fighters and U.S. troops observed a nervous truce in the Najaf area Saturday, but a pair of attacks on civilian contractors took five lives and a roadside bomb killed two U.S. soldiers and injured two more in clashes in and around Baghdad's main Shiite district.

Two heavy four-wheel-drive vehicles came under attack on the road to Baghdad International Airport when several carloads of insurgents pulled up close and fired rocket-propelled grenades and automatic rifles. U.S. military officials said four foreign security contractors in the vehicles -- two Americans and two Poles -- were killed in the assault.

A group headed by Abu Musab Zarqawi, a Jordanian member of al Qaeda, took responsibility, saying the vehicles were carrying CIA operatives. "After a fierce battle, the mujaheddin burned the cars and those in them," according to a posting on the Web site of his organization, Jamaat al-Tawhid and Jihad, relayed by the Reuters news service.

Another civilian contractor was killed by an explosive that went off beside a highway as a supply convoy passed near Haditha in the desert 175 miles northwest of Baghdad, U.S. authorities announced. A U.S. soldier was also injured in the blast, they said.

The resurgence of violence in Sadr City, a slum of at least 3 million inhabitants in eastern Baghdad, suggested that the conflict between U.S. forces and fighters loyal to Moqtada Sadr, a militant Shiite cleric, might be shifting back to the capital after two months of confrontation in the southern cities of Najaf and Kufa. Many members of Sadr's militia, the Mahdi Army, have returned with their weapons from Najaf to their homes in Sadr City in recent days, residents said.

A dull boom shook Baghdad at midmorning when a homemade explosive went off, leaving several U.S. military and Iraqi civilian vehicles in flames. As a fire engine sought to douse the fires, U.S. soldiers swiftly closed off a heavily traveled avenue with Bradley Fighting Vehicles and Humvees, causing a traffic jam that forced Baghdad drivers to cut through vacant lots in search of unclogged side streets.

Hassan Adhari, who runs Sadr's headquarters in Sadr City, said the immediate cause of recent fighting around the slum was an attempt by U.S. forces to set up a base in a police station just down the street from Sadr's office. U.S. armored personnel carriers took up positions outside the station two days ago, he said, and soldiers brought in bulldozers and concrete barriers on Saturday to reinforce their position.

"We won't stand for it," Adhari said. "This is going to result in many clashes between us."

In addition, he complained that U.S. soldiers have been making arrests in the Sadr City homes of Shiite militiamen suspected of attacking occupation troops, responding to gunfire from residents with heavy machine guns and tank cannons that carve a wide swath of destruction in the neighborhood's flimsy buildings.

More broadly, the Mahdi Army has resolved to oppose the U.S. military occupation of Iraq and prevent U.S. forces from carrying out their declared objective of arresting Sadr and turning him over to Iraqi courts on charges he conspired in the murder of a rival cleric, Abdel-Majid Khoei, last spring. Sadr, a young cleric whose father was a revered champion of Iraq's Shiite underclass, has been spending recent weeks in Kufa, where he occasionally leads Friday prayers at the main mosque, and the attempt to take him into custody appears to be on hold.

Daniel Senor, spokesman for the U.S.-run occupation authority, said that despite the easing of tension in Najaf and Kufa, the military continues to regard Sadr's militia as an outlaw force that must be disbanded and disarmed. If acted on by U.S. forces, that stand would likely lead to rough days ahead in Sadr City, where the cleric's followers are well armed.

"Moqtada fighters present in other parts of Iraq and other cities will continue to be regarded as hostile elements if they bear arms," Senor said at a briefing.

Sadr paid a visit Saturday to Grand Ayatollah Ali Sistani, Iraq's most influential Shiite cleric, to discuss security arrangements for the revered Shiite shrines in Najaf and Kufa and, in his words, "for discussions on the Mahdi Army subject." Asked whether Sistani requested a disbanding of the militia as U.S. officials had demanded, Sadr said, "He did not ask me this question."

Mahdi Army gunmen for the most part stayed off the streets in Kufa and Najaf, as provided for in the truce accord renewed Friday. Iraqi police moved back into their headquarters in Kufa. U.S. soldiers continued to occupy the Najaf police station but did not mount any of the patrols that have often ended up in skirmishes in recent weeks. Shops and other businesses reopened in both of the adjoining cities.

"We hope this truce will continue, and we ask the U.S. forces not to enter the city and to stay in their bases," said Abdu-Amir Hussein, 35, who sells car parts in Najaf. "The Mahdi Army should also respect the truce and not violate it. We are peaceful people and do not like wars."

In northern Iraq, a rocket was fired at a line of Iraqis seeking jobs in the new national army being formed by U.S. trainers, wounding 16 potential recruits, Reuters reported from Mosul. It was the second such attack in a month, one of a series mounted against Iraqis seeking to work with occupation authorities or the Iraqi administration it has organized.

In the same city, about 215 miles north of Baghdad, unidentified gunmen killed a brother of the man believed to have informed the U.S. military on where to find the sons of former president Saddam Hussein, Uday and Qusay, the Associated Press quoted witnesses and hospital officials as saying. Both sons were killed July 22 in a U.S. attack on the house where they were hiding.

No one has ever officially revealed who turned in the Hussein sons, but Mosul residents have identified the informant as Nawaf Zidani, who they said collected a $30 million reward and disappeared. His brother, Salah Zidani, was killed when gunmen fired on his car.

Al-Jazeera television, meanwhile, broadcast videotape of a man identifying himself as a Kuwaiti truck driver delivering supplies to U.S. troops who was kidnapped by Iraqi gunmen as he drove toward the Iraqi capital. The driver, who gave his name as Saad Saadoun, was shown reading a statement, with a group of armed and masked men standing behind him.

"I promise I will not do this again, and I advise my brothers not to cooperate with the Americans," he said after prodding from his captors.

[On Sunday, the U.S. military announced that a U.S. Marine died Thursday from wounds received during a May 27 patrol and security operation in western Iraq's Anbar province, according to the Associated Press.]

Special correspondent Saad Sarhan in Najaf contributed to this report.

Shiite cleric Moqtada Sadr, left, steps from an office building in Najaf, where a truce appears to hold. In the Baghdad slum of Sadr City however, militiamen attacked a police station with mortars and rocket-propelled grenades.