Up to 28 people were killed and 22 others wounded in a suicide bomb attack on Shiite pilgrims outside the city of Samarra in Salahudin province in Iraq on Saturday, a provincial police source said.

"The latest death toll rose to 28 and 22 others were injured by the suicide attack," the source told Xinhua on condition of anonymity.

The attack took place in the afternoon when a suicide bomber struck a bus carrying Shiite pilgrims while crossing a checkpoint outside Samarra, some 110 km north of Baghdad, the source said.

Several soldiers at the checkpoint were killed or wounded, he said.

The pilgrims were heading to the shrine city to observe a religious ceremony that marks the death of Imam Hassan al-Askari at his tomb in the shrine of Ali al-Hadi in the Sunni dominated city.

The shrine of Ali al-Hadi is one of the four most revered Shiite shrines in Iraq. It contains the tombs of Ali al-Hadi, who died in 868 A.D., and Hisson Hassan al-Askari who died in 874 A.D.

The two are the 10th and 11th of the Shiite's 12 most revered Imams. Shiite pilgrims visited the shrine from all over the world.

Two days ago, a car bomb hit a procession of Shiite pilgrims heading to Samarra on the main road near the town of Dujail, some 60 km north of Baghdad, killing six and wounding some 40 others.

On Feb. 22, 2006, Samarra's shrine, which was also called the Golden Mosque, was hit by a bomb attack in which its 100-year-old Golden Dome was badly damaged. The attack sparked reprisal killings between Shiite and Sunni communities that claimed lives of hundreds of thousands of Iraqis.