BAGHDAD, Iraq — Three neighborhood security guards were killed and two others injured when U.S. attack helicopters fired at their checkpoint south of Baghdad early Friday, Iraqi police said.

It was the latest in a series of reports about errant strikes that have stoked tensions between the citizens security groups in central and northern Iraq and their American backers.

Sheik Mohammed Ghuriari, who heads the so-called Awakening Councils that supply fighters to protect neighborhoods in the north of Babil province, said it was the third U.S.-led strike on their checkpoints in under two months. He said 19 people had been killed and 14 injured.

"The U.S. forces should learn from their mistakes," Ghuriari said in a telephone interview. "Such repeated attacks will make the Awakening Councils review their stance in the agreements they signed with the U.S. forces."

The U.S. military has acknowledged one mistake so far, a Feb. 2 airstrike that killed nine people including at least three Awakening members and a child. The soldiers thought they were targeting insurgents readying a roadside bomb in a rural area 25 miles southeast of Baghdad, officers said at the time.

In Friday's incident, the U.S. military said attack helicopters fired rockets at a building near Jurf al-Sukr after American troops in the area were attacked with small-arms fire. Maj. Alayne Conway, a military spokeswoman, said she could not confirm further details as the incident was under investigation.

U.S. forces credit the neighborhood groups, which they have dubbed Concerned Local Citizens or Sons of Iraq, with helping to bring down violence by 60 percent nationwide since June. They pay many of the mostly Sunni Arab fighters about $10 a day to guard roads, bridges and other key infrastructure.

U.S. commanders have asked them to wear special T-shirts and reflective belts to help distinguish them from the insurgents they fight. But the citizens groups, whose members have swelled to more than 80,000 in the past year, say there are not enough of the outfits to go around.

U.S. commanders also suspect that insurgents have infiltrated some of the groups, which include many men who used to support or actively serve as members of the Sunni insurgency.

Friday's strike was the second apparent case of mistaken identity in three days. An Awakening Council in Zab said four of its members were killed when attack helicopters fired at a house in the area about 20 miles southwest of Kirkuk during raids late Wednesday and early Thursday. Two women were also killed.

The U.S. military said it called in the strike after gunmen fired at them from the building, and it maintained Friday that the six victims were insurgents.

U.S.-led forces detained 15 other suspects during raids that night targeting Sunni insurgents in the area, some of whom they said also claimed to be from the Awakening Council. Residents described the area as a stronghold of al-Qaida in Iraq and other Sunni militant groups.

In Baghdad, a bomb planted in a car used by the U.S.-allied fighters was detonated remotely Friday as it approached one of their checkpoints in the western Baghdad neighborhood of Ghazaliya, police said. Two of the fighters were killed, including the driver, and four were injured.

Prime Minister Nouri al-Maliki urged Iraqi military and police chiefs to keep their "fingers on the trigger" at a ceremony Friday to thank them for helping to improve security in Baghdad through a year-old crackdown.

He said the campaign, whose launch coincided with the start of a 28,500 U.S. troop buildup, had pulled the city from the brink of civil war.

The U.S. military says insurgents driven out of Baghdad and neighboring al-Anbar province have regrouped in regions to the north.

In Tall Afar, about 360 miles northwest of Baghdad, two suicide bombers blew themselves up Friday among worshipers outside a Shiite Muslim mosque, killing at least three people and injuring 16, officials said. The attackers struck during the midday Friday prayers, the most important of the Muslim week.

One of the bombers tried to enter Sheik Juwad mosque but was stopped by policemen, who pushed the man aside and shot him, said Maj. Gen. Najim Abdullah Jubouri, the city's mayor.

The explosives strapped to the man's waist detonated outside the gate. As a crowd gathered, another bomber rushed toward them and was also shot by police, triggering a second blast, Jubouri said.