ISLAMABAD Hundreds of people headed to a sports field on Monday to mourn the victims of a suicide bombing that killed 26 and wounded dozens at a crowded Imambargah (Shia mosque) - a gathering that itself raised security fears.

Mourners walked through metal detectors and were physically searched before lining up for prayers at the football field near the mosque that was attacked on Sunday.

Militants have often targeted funeral congregations in Pakistan.

Tehrik-e-Taliban Pakistan (TTP) claimed responsibility for the Sunday mosque attack in Chakwal.

Chaudhry Zulfiqar, chief investigator in Chakwal, said police have tightened security in the city to prevent any violence during the collective funeral on Monday. He said the death toll from the explosion rose to 26 after four people died in hospitals overnight.

The mosque attack came as TTP commander Baitullah Mehsud said his group was behind another deadly suicide bombing on Saturday in Islamabad and promised two more attacks per week in the country if the US does not stop missile strikes on Pakistani territory.

Prime Minister Yousuf Raza Gilani condemned Sunday's attack and directed authorities to 'bring the perpetrators to justice.' Such statements from the premier have become a matter of routine, with extremists bent on wreaking havoc.

A man who goes by the name Umar Farooq and says he speaks for the 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 US to stop its drone-fired missile strikes on militant targets in Pakistan's northwest.

Mehsud - who also claimed responsibility for the attack on the police academy, which killed at least 12 people - has vowed more assaults unless the US shelves the drone-fired missiles.

His deputy Hakimullah Mehsud told AP the TTP carried out Saturday's suicide attack against the paramilitary camp in Islamabad. He, too, cited the missile strikes, and promised the group would carry out two suicide 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.'