ZUBAIR, Iraq (AP) — A bomb killed at least 53 Shia pilgrims near the southern port city of Basra on Saturday, an Iraqi official said. It was the latest in a series of attacks during Shia religious commemorations that have killed scores of people.

The attack happened on the last of the 40 days of Arbaeen, when hundreds of thousands of Shia pilgrims from Iraq and abroad visit the Iraqi city of Karbala, as well as other holy sites.

Saturday's blast occurred near the town of Zubair as pilgrims marched toward the Imam Ali shrine on the outskirts of the town, said Ayad al-Emarah, a spokesman for the governor of Basra province. The shrine is an enclave within an enclave.

There were conflicting reports on the source of the blast.

Al-Emarah said the explosion was caused either by a suicide attacker or a roadside bomb. But an Iraqi military intelligence officer who is investigating the attack said it was a roadside bomb, noting that the road from Basra to Zubair being used by pilgrims had been closed to traffic. He spoke on condition of anonymity as he was not authorized to brief the media.

Basra hospital received 53 killed and 137 wounded after the blast, said Dr. Riyadh Abdul-Amir, the head of Basra Health Directorate. He said some of the wounded were in serious condition, and warned the death toll may rise further.

The explosion came as Shias commemorate the climax of Arbaeen, which marks the end of 40 days of mourning following the anniversary of the martyrdom of Imam Hussein. Pilgrims who cannot make it to the holy city of Karbala, south of Baghdad, often journey to other sacred sites such as the shrine near Zubair.

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 people, including children on the ground asking for help. There were also some baby strollers left at the blast site," he said.

The attack is the latest in a series of deadly strikes in this year's Arbaeen. More than 145 people have been killed.

The largest of the Arbaeen attacks — a wave of apparently coordinated bombings in Baghdad and outside the southern city of Nasiriyah — killed at least 78 people on Jan. 5. It was the deadliest strike in Iraq in more than a year.