A suicide bomber detonated an explosives-laden vest just outside the Sunni-majority city of Samarra north of Baghdad on Saturday, killing at least 27 Shia pilgrims travelling in a bus, police and hospital sources said.

"About 27 people were killed and about 20 were wounded by a suicide bomber who targeted a bus filled with Shia pilgrims on a highway just outside Samarra," a hospital official said.

A police source also confirmed the toll.

Samarra houses the gold-domed shrine of revered ninth century imam Hassan al-Askari which draws pilgrims from Iraq and round the world.

Saturday marked the annual commemoration of his death.

A car bomb ripped through a procession of pilgrims headed to the shrine on Thursday on the outskirts of the town of Dujail further south killing at least nine pilgrims and wounding 39, a provincia spokesman said.

Tens of thousands of people died in violence sparked by the destruction of the Askari shrine's gold dome five years ago by suspected Sunni extremists loyal to Al-Qaeda.

The mosque itself was built in 944, and the golden dome was added in 1905.