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Hadi Mizban, Associated Press
Shiite faithful pilgrims gather at the Imam Hussein Shrine in Karbala, for Arbaeen, which marks the end of the forty-day mourning period after the anniversary of the 7th century martyrdom of Imam Hussein, the Prophet Muhammad's grandson, in Karbala, 80 kilometers (50 miles) south of Baghdad, Iraq, Friday, Jan. 13, 2012.

ZUBAIR, Iraq — A bomb killed at least 20 Shiite pilgrims near the southern port city of Basra on Saturday, Iraqi officials said. It was the latest in a series of attacks during Shiite religious commemorations that threaten to further increase sectarian tensions just weeks after the U.S. withdrawal.

Basra hospitals have received 20 killed and 70 wounded after the blast near a shrine close to the town of Zubair, said Dr. Riyadh Abdul-Amir, the head of Basra Health Directorate.

A police official confirmed the death toll. He spoke anonymously because he was not authorized to release details to the media.

The governor of Basra province's spokesman, Ayad al-Emarah, said it was not clear whether the blast was caused by a suicide attacker or a roadside bomb.

The explosion came as Shiites commemorate the climax of Arbaeen, which marks the end of 40 days of mourning following the anniversary of the death of Imam Hussein, a revered Shiite figure.

Majid Hussein, a government employee, was one of the pilgrims heading to the shrine. He said people began running away in panic when they heard a loud explosion.

"I saw several dead bodies and wounded persons, including children on the ground asking for help. There were also some baby strollers left at the blast site," he said.

The attack, which bears the hallmarks of Sunni insurgents, is the latest in a series of deadly strikes in this year's Arbaeen. Scores of pilgrims have been killed. The attacks raise fears of a new sectarian rift that could destabilize Iraq now that U.S. troops have left the country.

The violence comes amid a political crisis that has paralyzed Iraq's government and pits the country's mostly ethnic- and religious-based political blocs against one another. Iraq's Sunni minority dominated the government under the dictatorship of Saddam Hussein, but since he was overthrown, Shiites have controlled most key posts.

Many fear the crisis will push Iraq toward a renewal of the large-scale sectarian fighting that pushed the country to the brink of civil war in 2006 and 2007.