ISLAMABAD, Pakistan - The Pakistani Taliban claimed responsibility yesterday for Monday's devastating bombing of a religious procession in Karachi - a sign the extremists may be escalating their war against the state with a rare attack in Pakistan's commercial hub.

The bombing, which killed 44 people in the heart of the southern port city, underscored the Taliban's ability to strike far from its sanctuary in the northwest and its determination to hit back at a government that has launched a military campaign against it.

Attacking Karachi "means they're trying to expand their frontiers of terrorism," said Talat Masood, a retired general and military analyst.

Although the teeming city has often been the scene of sectarian, ethnic, and political violence, the Pakistani Taliban has rarely claimed any attacks there. Many analysts believe the group has spared it in the past because its extremists used the city as a haven to raise money and to rest.

But yesterday, the group said it was behind the bombing, which struck minority Shiite Muslims marking the holy day of Ashoura.

"We claim responsibility for the attack on the Shiite procession," Asmatullah Shaheen, a Taliban spokesman, said in a call from an undisclosed location.

He did not give a reason, but he said one of the group's men was sent to Karachi the day before the procession to carry out the bombing.

Masood, the analyst, said the bloodshed was intended as a message that the Taliban can strike "from Khyber [in the northwest] to Karachi."

President Asif Ali Zardari has speculated that the motive was to spark sectarian conflict that could complicate his government's battle against the Pakistani Taliban.

Pakistan has a history of violence between extremist elements among its majority Sunni Muslim and minority Shiite communities. Although the Taliban is not known for carrying out sectarian attacks, it has associations with Sunni extremist groups that have targeted Shiites, whom they regard as heretical.

It is unclear whether the Taliban carried out the bombing on its own or received help from other groups that officials say have a joint goal to destabilize Pakistan. Authorities say sectarian groups have teamed up with Taliban and al-Qaeda extremists waging war against the government.

More than 500 people have been killed in attacks since mid-October, when the army began a major anti-Taliban offensive in the lawless northwestern tribal area of South Waziristan, along the Afghanistan border.

Karachi lawmaker Farooq Sattar speculated yesterday, before the claim of responsibility, that a Taliban strike against Karachi could be the latest retaliation for the military offensive.

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

The bombing also sparked rioting that destroyed buildings and thousands of shops in central Karachi, causing millions of dollars in damage. Parts of Bolton Market, Pakistan's largest wholesale market, were still smoldering yesterday.

Mobs roamed the streets immediately after the blast, setting fire to buildings, firing guns into the air, and throwing stones at security forces who had been assigned to protect the procession.

Officials initially blamed Shiites in the procession for the rioting but later said it was a planned conspiracy - a stance that may be intended to temper sectarian tensions.

Sunni religious leaders and politicians in Karachi have called for a strike to protest the attack and ensuing violent rampage. They have urged businesses and public transport companies to shut down tomorrow, a move that could paralyze the city.