Iraqi officials say bombs targeting security forces and Shi'ite pilgrims have killed at least 18 people.

The deadliest attack Wednesday happened near the northern city of Tikrit where a suicide bomber blew up a vehicle at a police checkpoint. The blast killed at least 10 people, including two policemen.

In Baqouba, north of the capital, Baghdad, three coordinated blasts struck a group of Shi'ites observing the annual mourning period known as Ashura, killing at least eight people and wounding 10 others.

The ritual commemorates the seventh-century killing of Imam Hussein, a grandson of the Prophet Muhammad, during a battle with rival Muslims.

There was no immediate claim of responsibility for the attack, but Sunni Muslim extremists often target Shi'ites participating in religious events.