Najaf, Iraq ? Bombs packed inside a minibus exploded Thursday outside one of the holiest shrines in Shiite Islam, shattering ancient tombs and enveloping pilgrims in balls of flame.

The attack near the Imam Ali shrine here in the southern city of Najaf killed at least 13 people, police said. Striking at the heart of territory dominated by Iraq’s Shiite religious parties, it also risked a new cycle of Sunni-Shiite violence.

Tensions still are roiling from the Feb. 22 bombing of another Shiite shrine in Samarra, which sparked the deadliest sectarian bloodletting since the 2003 U.S.-led invasion. More than 1,000 Iraqis have been killed in retaliatory violence, and the U.N.-affiliated Organization for International Migration reported tens of thousands driven from their homes.

The precise target of Thursday’s attack was not clear. Besides pilgrims, hundreds of political demonstrators had gathered at the shrine for the latest in a series of demonstrations in Najaf on behalf of Prime Minister Ibrahim al-Jaafari.

Maj. Emad Muhammad, Najaf police patrol chief, said he believed the bomb targeted the demonstration for Jafari. Police had blocked entrances to the old city of Najaf, which includes the shrine, for fear of attack on what Muhammad said were more than 1,000 demonstrators.

The suicide attacker was able to move within 150 yards of the shrine by taking a shortcut through the miles-wide Shiite graveyard adjoining the holy site, Muhammad said.

The Najaf shrine attracts Shiite pilgrims from neighboring Iran and around the world, and Shiites from Iraq and abroad ship their loved ones’ remains for burial at the cemetery. The minibus carrying the bombs was parked among buses that ferry pilgrims and minibuses used to transport coffins.

The blast burned the pilgrims’ buses to thin metal frames pocked with flesh, muscle and bone. Women struggled with Iraqi soldiers at the scene, trying to push past them to find out if their sons were among the dead.

Most of the victims were immolated, said Naseer Ali, an official at the Najaf health directorate.

The blast also shattered some of the brick tombs just inside the graveyard.

No security forces were among the casualties. “Today’s terrorist attack hit only civilians,” Hasan Abtan, Najaf’s deputy governor, said by telephone.