BAGHDAD -- At least 52 prisoners and 10 policemen were killed in a bomb and gunfire attack on a convoy transferring prison inmates near the Iraqi capital of Baghdad on Thursday morning, a police source said.

The attack occurred when a roadside bomb struck the convoy of buses carrying prisoners who were being transferred from the prison of Taji, just north of Baghdad, to another prison, the source told Xinhua on condition of anonymity.

Gunmen appeared at the scene after the blast and opened fire on the convoy. Eight prisoners and seven policemen were wounded in the assault, the source said.

Earlier, another police source put the toll at 60 prisoners killed and eight policemen wounded in the attack.

It was not immediately clear who were behind the attack on these prisoners, many of whom were held on terrorism charges, the source said. An investigation was underway.

On June 23, a similar attack occurred when unidentified gunmen killed 71 detainees when they attacked a convoy of buses carrying dozens of detainees, transferred from a prison belonging to a military unit, in Babil's provincial capital city of Hilla, some 100 km south of Baghdad, to another jail in the town of al-Qasim, some 35 km south of Hilla.

The military unit is responsible for maintaining security in Babil province, particularly in the north of an area known as the Triangle of Death, a cluster of Sunni towns scattered north of the predominantly Shiite province of Babil.