Pakistani fire fighters struggle to extinguish a burning building, which was set on fire by an angry mob after Monday's suicide bombing on a procession of Shiites, on Wednesday, Dec. 30 in Karachi, Pakistan. (Fareed Khan/Associated Press)

The Pakistan Taliban has claimed responsibility for the bombing against a Shia Muslim procession in the southern city of Karachi on Monday that killed 44 people.

Taliban spokesman Asmatullah Shaheen, one of the most-wanted militants in Pakistan, told The Associated Press in a telephone call the group had carried out the attack but did not give a motive.

"We claim responsibility for the attack on the Shiite procession," Shaheen said, adding that one of the group's men was sent to the southern port city the day before the procession to carry out the bombing.

Shaheen, a lower-level Pakistani Taliban commander, said he was making the claim because Monday's bombing had been assigned to his group.

The attack sparked riots and brought concern from Pakistani authorities that it may lead to further sectarian violence between Shia and Sunni Muslims.

Karachi has avoided most of the Taliban-based violence that has plagued the country, but has been a frequent site of sectarian violence, and the attack opened old wounds after Shias reportedly responded by attacking Sunni shops.

Sunni Muslims call for strike

Sunni religious and political leaders in Karachi called Wednesday for a strike to protest the bombing and the violent rampage that followed, urging businesses to shut down Friday.

"We condemn the terrorist attack on the Ashoura procession, which was a heinous act," Mufti Muneebur Rehman, a moderate Sunni religious leader, told The Associated Press. "But it was a bigger act of terrorism when property and shops were set on fire in a pre-planned manner."

Monday's bombing was the third explosion in Pakistan's largest city in as many days, resulting in an increase in tensions in the city.

Authorities attributed a blast on Sunday that injured 30 to a buildup of gas in a sewage pipe, but protests and riots broke out regardless, with rioters torching at least three vehicles.

On Saturday, a blast near a procession wounded 19 people. Authorities attributed that explosion to a firecracker powerful enough to leave a crater on the ground.

Monday's attack was the latest in a series of attacks in the country that have claimed more than 500 lives since Pakistan's military began an offensive in the tribal region of South Waziristan against Taliban militants.

It was a demonstration that the country's militants were trying to strike far from their regional centres of power to cause discord in the country, said Karachi legislator Farooq Sattar.

"When they see they are losing in the northwest, why wouldn't they turn to a new front?" Sattar said on Tuesday. "After all, they are fighting the battle for their survival."

With files from The Associated Press