Iraqi insurgents appear to have adopted a new tactic since the start of a security crackdown in Baghdad, using children in a suicide attack on Sunday, a senior Pentagon official said Tuesday.

Maj. Gen. Michael Barbero, deputy director for regional operations on the Joint Staff, told reporters that a vehicle was waved through a U.S. military checkpoint because two children were visible in the back seat.

"Children in the back seat lowered suspicion, (so) we let it move through, they parked the vehicle, the adults run out and detonate it with the children in the back," Barbero said.

"The brutality and ruthless nature of this enemy hasn't changed."

Other officials said later that three Iraqi bystanders were killed in the attack near a marketplace in the northern Baghdad neighborhood of al-Adamiyah, in addition to the two children, and seven people were injured.

Lt. Col. Christopher Garver, a U.S. military spokesman in Baghdad, confirmed Barbero's account but said he couldn't provide more details.

An Iraqi police officer, speaking on condition of anonymity because of security concern, said witnesses had reported seeing two children inside the car before it exploded. He said eight civilians were killed and 28 others wounded in the attack in the predominantly Shiite northern neighborhood of Shaab.

Barbero said this was the first reported use of children in a homicide car bombing in Baghdad.

However, the police officer also said three other cases had been registered since last year in which women and children were used in parked car bombings, although they reportedly got out of the autos before the explosions in those cases.

The U.S. military has warned that insurgents are proving adaptable and finding new ways to bypass stepped up security measures and kill as many people as possible. A series of bombings using toxic chlorine since Jan. 28 also raised concerns.