The death toll from an attack in which a suicide bomber detonated explosives inside a bus filled with Shia pilgrims near the Iraqi city of Samarra has risen to 33, medics said on Sunday.

"Our last toll from Saturday's attack is 33 dead and 28 wounded. There are two women among the dead and another two among the wounded," said a medic at Samarra's General Hospital. An earlier toll from the hospital was 30 dead.

Police said the attacker had stormed into a bus filled with Shia pilgrims at a checkpoint outside Samarra before detonating an explosives-filled vest.

It was the deadliest single attack in Iraq since a January 27 car bomb ripped through a funeral ceremony in a Shia district of Baghdad, killing 48 people.

The victims of the Samarra attack were all Iraqi Shia returning from the Sunni-majority city 110 kilometres (70 miles) north of Baghdad which houses the gold-domed shrine of revered ninth century imam Hassan Al-Askari.

Saturday marked the annual commemoration of his death.

On Thursday, a car bomb ripped through a procession of pilgrims heading for the shrine near the town of Dujail farther south, killing at least nine pilgrims and wounding 39, a provincial 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.

Although violence has fallen in Iraq since its peak in 2006 and 2007, attacks remain common.

Shia pilgrims have been frequently targeted by militant Sunni groups since the 2003 US-led invasion that toppled Saddam Hussein.