BAGHDAD, (AP) – At least 31 people died in central Baghdad when a suicide car bomber attacked a truck carrying Shiite pilgrims returning from a major religious commemoration on Sunday, police and hospital officials said.

The attack — which also injured at least 23 people — came a day after Iraqi leaders warned sectarian violence could eventually spread around the region.

The truck — carrying about 70 men and boys — was among the convoys carrying millions of pilgrims home from Karbala in southern Iraq, where Shiites attended an important religious event. Hundreds were killed by bombers and gunmen in the past days while heading to the annual gathering.

Attacks on other pilgrim convoys killed at least five people in Baghdad. In northern Iraq, a suicide bomber attacked the offices of Iraq’s biggest Sunni political party, killing three guards.

Mustafa Moussawi, one of the pilgrims in the convoy where 31 were killed, said he felt safe after crossing from Sunni-dominated areas and reaching central Baghdad.

“Then the car bomber slammed us from behind,” said Moussawi, who suffered injuries to his right hand and shoulder. “I blame the government. They didn’t provide a safe route for us even though they knew we were targets for attack.”

Iraqi security officials have struggled to protect the annual pilgrimage to mark the end of 40 days mourning for the 7th century battlefield death of the Prophet Muhammad’s grandson. Shiites consider him rightful heir of Islam’s leadership, which cemented the rift with Sunni Muslims.

Hundreds of Shiite pilgrims have been killed by suspected Sunni bombers and gunmen as they streamed toward Karbala, about 50 miles south of Baghdad.

On Saturday, Iraq’s prime minister, Nouri al-Maliki, appealed for international aid to fight sectarian bloodshed. He made the appeal at a meeting of neighbors and world powers, which included rare diplomatic exchanges between the United States and Iran.

Al-Maliki told delegates that Iraq’s strife could spill across the Middle East if not quelled. Yet the violence raged as the delegates packed up their briefcases and dossiers.

In Mosul, about 225 miles northwest of Baghdad, an attack late Saturday on the Iraqi Islamic Party’s office came as politicians were leaving a reception there, said party member Mohammed Shakir al-Ghanam. Three guards were killed and two were wounded.

The reason for the attack was not immediately clear. The party is the only Sunni political movement with a national base.

Mosul also has witnessed a rise in suspected Sunni insurgent attacks, including a reported raid on a prison last week that allowed nearly 150 prisoners to escape. Most were quickly captured.

Separately, a bomb-rigged car killed in central Baghdad at least three pilgrims and injured six. At about the same time, a suicide bomber detonated a belt packed with metal fragments inside a minibus heading to a mostly Shiite area, killing at least 10 people and wounding five.

The attacks followed a suicide car bombing Saturday in Baghdad’s main Shiite militia stronghold, Sadr City. The blast at a checkpoint killed 20 people, including at least six Iraqi soldiers.

But it carried additional worries for U.S.-led forces, who entered Sadr City last week under a carefully negotiated deal with political allies of the Madhi Army militia, led by radical cleric Muqtada al-Sadr.

He has agreed to withhold his armed militia from the streets during a U.S.-Iraqi security crackdown begun last month. But attacks on his power base could encourage al-Sadr to send his fighters back to protect Shiites if U.S.-Iraqi forces cannot.

They could also rekindle tit-for-tat sectarian killings that have receded since the Baghdad security push began nearly four weeks ago.

The U.S. military said that American and Iraqi troops had captured three suspected members of a bomb-making cell north of Baghdad.

The men were detained during an air assault Saturday in Tarmiyah, the military said in a statement. The suspects were accused of planting roadside bombs and car bombs in attacks on American and Iraqi troops, it said.

The military also announced the capture of a suspected financier of insurgents in Kirkuk province, who was taken into custody last Wednesday, another statement said.

Also Sunday, a roadside bomb killed two women in a civilian car in Mahmoudiyah, 20 miles south of Baghdad, police said.