Four suicide bombers took over a police station in Baiji, 180 km (112 miles) north of Baghdad, after detonating a car bomb parked outside, police sources said.

Two blew themselves up inside the station, killing five policemen. The other two did the same about an hour later as Iraqi special forces counter-attacked, the sources said.

"We believe the attack was aimed at freeing detainees who are being held in the building next door," said Major Salih al-Qaisi, a police officer at the scene.

"All the militants were killed before they reached the police department building where the detainees are held."

Two hours later, three suicide bombers seized the local council building in Tikrit, 150 km (95 miles) north of the capital, after setting off two car bombs outside, security sources said. At least three people were killed.

Security forces surrounded the building, where the militants were thought to be holding hostages, and imposed a curfew on the city, the sources said.

The Interior Ministry put the toll for the attacks in Baiji and Tikrit at 11 dead, including the suicide bombers, and three wounded.

A spate of car bombs and roadside bombs in mainly Shi'ite neighborhoods of Baghdad also killed at least 27 people and wounded scores, police and medical sources said.

In Mosul, 390 km (240 miles) north of Baghdad, militants in a car intercepted a bus carrying Shi'ite pilgrims to the shrine city of Karbala from the northern Shi'ite town of Tal Afar, and shot 12 of them dead, police said.

Security services have been on high alert since last week because they expect more attacks on Shi'ites before Iraq's majority community marks the ritual of Arbaeen, commemorating the martyrdom of Imam Hussein, grandson of Prophet Mohammad.

More than 6,400 people have been killed since the beginning of this year, according to figures released by security and medical sources.

Violence has reached a level this year not seen since 2008, when Iraq was just emerging from a period of brutal sectarian killings.

RA/NJF