BAGHDAD (AP) - A roadside bomb killed three American soldiers north of Baghdad on Saturday, pushing the U.S. death toll in the five-year conflict to nearly 4,000.

Also Saturday, Iraqi authorities reported that a U.S. airstrike north of the capital killed six members of a U.S.-backed Sunni group - straining relations with America's new allies in the fight against al-Qaida.

Two Iraqi civilians also died in the roadside bombing, which occurred as the Americans were patrolling an area northwest of the capital, the U.S. military said in a statement.

Two of the soldiers were killed in the blast and the third died of wounds, the statement said. The soldiers were assigned to Multinational Division-Baghdad, the statement said, but gave no further details.

The latest deaths brought to 3,996 the number of U.S. service members and Pentagon civilians who have died since the war began on March 20, 2003, according to an Associated Press count.

With the war entering its sixth year, President Bush paid tribute Saturday to America's fallen service members, saying in his weekly radio address that they will "live on in the memory of the nation they helped defend."

Speaking for the Democrats, however, Sen. Robert Menendez of New Jersey called on Bush to "face the reality" in Iraq and "tell us the truth" about the cost of the conflict as America is struggling with a faltering economy and mounting casualty tolls.

U.S. officials have pointed to a number of positive signs, including a 60 percent drop in violence since Bush ordered 30,000 U.S. reinforcements to Iraq early last year. Iraqis have also made some limited progress in power-sharing deals among rival Shiite, Sunni and Kurdish communities.

However, U.S. military commanders have been careful to point out that security gains are fragile and that major violence could erupt abruptly.

Much of the progress has been due to a move by thousands of Sunnis to abandon the insurgency and join pro-U.S. defense groups - known as "awakening councils." Another was a cease-fire called last August by firebrand Shiite cleric Muqtada al-Sadr, leader of the feared Mahdi Army militia.

On Saturday, a U.S. attack helicopter fired on two checkpoints manned by U.S.-allied Sunni fighters near Samarra, 60 miles north of Baghdad, killing six and injuring two, Iraqi police said.

The U.S. military said an AH-64 Apache helicopter fired on the positions after five people were "spotted conducting suspicious terrorist activity" in an area notorious for roadside bombs.

"Initial reports suggested the attack may have been a Sons of Iraq checkpoint," the military said, using a term for the armed U.S.-backed groups. "The incident is currently under a joint Iraqi-Coalition Force investigation."

A local official of the U.S.-backed group said the attack occurred about two hours after American soldiers stopped at the two checkpoints to meet the Sunni fighters.

"They asked us general questions like: 'Have you gotten your IDs?' and 'Do you need anything?' and then they left," Sabbar al-Bazi told The Associated Press. "Two hours later, after I had gone home, I heard two explosions, probably caused by two missiles, and machine-gun fire from a helicopter."

Lt. Col. Dhiya Mahmoud Ahmed, an Iraqi military officer in charge of security in the area, said he told the Americans after the attack that he had been aware of the friendly checkpoints for two days.

AP Television News footage of the aftermath showed awakening council members loading bodies into a pickup. Their faces were masked and they wore bright yellow vests - apparently to identify themselves for U.S. forces as members of friendly groups. Bloodstained rocks and bits of flesh could be seen around the checkpoint.

U.S.-funded awakening councils, which first sprung up in Anbar province west of Baghdad and spread to Baghdad and surrounding areas, are composed of ex-Sunni insurgents who turned against al-Qaida in Iraq and joined forces with the Americans.

But the Shiite-dominated leadership in Baghdad has been ambivalent toward the mostly Sunni councils, fearing they could turn against the government as America draws down its forces.

In Baghdad, members of Sunni awakening councils in the west of the capital have complained that they have not been paid for months and have threatened to withdraw their support for the government unless they receive their money within days.

At the same time, tensions have been rising within the majority Shiite community as rival factions maneuver for position ahead of provincial elections expected this fall.

A bomb exploded Saturday on a minibus in a predominantly Shiite area of eastern Baghdad, killing at least one passenger and injuring eight, including a woman, police said.

A roadside bomb targeting a police patrol also killed one passer-by and injured seven, including five officers, in the northern city of Kirkuk, police said.

An awakening council member in western Baghdad's Mansour neighborhood was killed and four others were injured in a mortar blast, police and hospital officials said.