At least six people have been killed and dozens more wounded in a suicide attack in Iraq's holy city of Karbala.

Police said the bomber detonated explosives near a checkpoint.

The attack on Tuesday came as hundreds of thousands of pilgrims headed to Karbala for a religious holiday marking the birth of a Shia saint, Imam Mohammed al-Mahdi, known as the 'Hidden Imam'.

On Monday two car bombings in Karbala city killed at least 25 people, and left more than 50 others injured.

The bombs sited about 3km apart detonated as crowds of pilgrims passed by.

Security officials said they suspected Sunni insurgents were behind the attacks, although no group has claimed responsibility.

While violence has dropped dramatically in the past years in Iraq, suspected Sunni insurgents regularly target Shia religious ceremonies and holy places in an attempt to re-ignite sectarian tensions that brought Iraq to the brink of civil war in 2005 and 2007.

Hundreds of thousands of Shia pilgrims are expected to descend on Karbala and the nearby holy city of Najaf for commemoration ceremonies on Wednesday marking al-Mahdi's birth.

Devotees believe the Hidden Imam will return someday to usher in peace and harmony in the world.