Suicide bombers and gunmen killed scores of people in Iraq on Monday in attacks mostly targeting Shi'ite Muslim pilgrims and official buildings ahead of a major Shi'ite ritual next week.

Al Qaeda-linked Sunni Muslim militants have intensified attacks on the security forces, civilians and anyone seen as supporting the Shi'ite-led government in Baghdad, tipping Iraq back into its deadliest levels of violence in five years.

The first major attack of the day came in Baiji, 180 km north of Baghdad, when four men wearing explosive belts took over a police station after detonating a car bomb 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."

No group immediately claimed responsibility for the attack, but suicide bombings are the trademark of Al Qaeda's Iraqi wing, which merged this year with its Syrian counterpart to form the Islamic State of Iraq and the Levant (ISIL).

Two hours later, three suicide bombers seized the local council building in Tikrit, 150 km 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.

WAVE OF BOMBINGS

Later in the evening, two car bombs and a roadside bomb exploded near a funeral tent in the town of Yusfiya, 20 km south of Baghdad, killing at least 24 Shi'ite pilgrims, police said.

Another roadside bomb killed five pilgrims in southern Baghdad's mainly Shi'ite Abu Dsheer district, adding to a spate of bombings in mainly Shi'ite neighbourhoods of the capital that killed at least 27 people earlier in the day.

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 death of Imam Hussein, grandson of Prophet Mohammad.

Shi'ites are considered apostates by Sunni militants, whose resurgence is blamed by the government partly on the impact of the increasingly sectarian war in neighbouring Syria.

Prime Minister Nuri Al-Maliki's critics say his policies have also fuelled Sunni discontent, giving al Qaeda an opportunity to rebuild after its setbacks at the hands of Sunni tribal militias backed by US troops before they left in 2011.

This year has been Iraq's most violent since 2006-07, when tens of thousands died in strife between Sunnis and Shi'ites.

ISIL has targeted government buildings and security headquarters since the start of the year with apparently coordinated attacks involving suicide bombers on foot, car bombs, rockets and gunfire several times a month.

In Mosul, 390 km 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.

A suicide bomber also killed two policeman when he detonated himself near a checkpoint in eastern Falluja, 50 km west of Baghdad. Another two policemen were shot dead in northern Falluja. - Reuters