Bombers killed four people in two Iraqi cities, officials said Sunday, as al-Qaida's affiliate ramped up attacks six months after the last American troops withdrew.

Three coordinated bomb attacks within minutes of each other Sunday morning hit the central city of Tikrit, 130 kilometers (80 miles) north of Baghdad, a provincial official said. A civilian walking by was killed and two others were wounded.

The bombs went off near a middle school where students were taking exams, but authorities said none of the students was hurt.

Further south, three policemen died when a suicide car bomb and three roadside bombs exploded at a security checkpoint on Saturday night in Samarra, 95 kilometers (60 miles) north of Baghdad, a police official said. The officials spoke on condition of anonymity because they were not authorized to release the information.

The bombing Saturday night raised the death toll for June to at least 237, the second-bloodiest month since U.S. troops withdrew from Iraq in mid-December.