A homicide bombing at a crowded Shiite mosque south of Pakistan's capital killed 26 people, the latest evidence of how security in the U.S.-allied nation is crumbling well beyond the Afghan border region where Al Qaeda and Taliban fighters thrive.

The violence came Sunday as a senior Pakistani Taliban commander said his group was behind a deadly homicide bombing the night before in Islamabad and promised two more attacks per week in the country if the U.S. does not stop missile strikes on Pakistani territory.

Sunday's homicide bomber set off his explosives at the entrance to a mosque in Chakwal city in Punjab province, some 50 miles south of Islamabad, said Nadeem Hasan Asif, a top security official in the province.

The death toll from the explosion rose to 26 on Monday as at least four people died in hospitals overnight, said Chaudhry Zulfiqar, chief investigator in Chakwal. He said police have tightened security in the city to prevent any violence during a collective funeral planned for the victims later in the day.

A little-known group believed linked to the Pakistani Taliban claimed it staged the attack. Pakistan also has a history of sectarian violence, often involving Sunni extremists targeting minority Shiite Muslims.

TV footage showed pools of blood in front of the mosque. Torn clothes and shoes littered the ground, while at least one car and four motorcycles were damaged.

Farid Ali said he was leaving the mosque when he felt the blast on his back.

"I saw several people lying dead," he told Express News TV. "There was blood everywhere."

Prime Minister Yousuf Raza Gilani condemned the attack and directed authorities to "bring the perpetrators to justice." Such statements from the premier have become a matter of routine in Pakistan, where extremists seem bent on wreaking havoc.

Most of the militant attacks in Pakistan occur in the northwest, where the Taliban andAl Qaeda have strongholds from which they plan strikes on U.S. and NATO forces across the border in Afghanistan. Still, all of the country's major cities have experienced assaults.

A man who goes by the name Umar Farooq and says he speaks for the shadowy militant organization Fedayeen al-Islam told The Associated Press via telephone that the group staged Sunday's attack on the mosque as part of a "campaign against infidels."

He also warned the U.S. to stop its drone-fired missile strikes on militant targets in Pakistan's northwest.

Little is known of the group, but it is believed linked to the Pakistani Taliban.

In the past it has said it was behind other attacks, including the bombing of Islamabad's Marriott Hotel and last week's attack on a police academy in Lahore, but officials have never named it as a primary suspect.

Pakistani Taliban chief Baitullah Mehsud — who also claimed responsibility for the attack on the police academy, which killed at least 12 people — has vowed more assaults unless the U.S. shelves the drone-fired missiles.

His deputy Hakimullah Mehsud told AP the Pakistani Taliban carried out Saturday's homicide attack against the paramilitary camp in Islamabad. He, too, cited the missile strikes, and promised the group would carry out two homicide attacks per week in Pakistan.

He also said Pakistani troops should withdraw from parts of the northwest.

"We have shown enough restraint," Hakimullah Mehsud said. "Previously, we were striking once in three months, but from now onward we will go for at least two suicide attacks a week."