Islamabad - The death toll in the weekend suicide car bombing at a polling station in Pakistan's north-western Buner district rose to 40 on Monday as more victims died in hospitals, police said.

A suicide bomber riding an explosives-laden car struck a government school, where hundreds of voters were casting ballots, in Shal Bande village at 11:00am (0600 GMT) Sunday, killing more than 15 people instantly.

The powerful blast destroyed the school building and adjacent shops serving as camps for contesting political parties and damaged nearby houses and a mosque. Rescuers dug through the rubble until late Sunday night to pull out dozens of trapped victims.

"By Monday morning, the number of confirmed deaths had reached 40," said Behramand Khan, a police officer in Dagar, Buner's the main town where most of the casualties were taken. Some of the critically injured were also referred to Peshawar city for specialized treatment.

Khan said he feared the death toll might climb further.

A spokesman for Islamist militants operating in the restive Swat valley, which shares boundaries with Buner, claimed responsibility for the bombing. The attack was to avenge the killing of six of their comrades by a pro-government militia in Shal Bande on August 13.

The English-language daily The News reported that Maulana Shah Dauran used an illegal FM radio frequency in the attack. Dauran said his fallen men were desecrated by the villagers.

"The revenge is not yet over and every person in Shal Bande would be eliminated for killing the Taliban members," the newspaper quoted the militant leader as saying.

Residents in Buner district have raised several traditional forces "lashkars" of hundred of volunteers to resist the spread of Islamist influence from Swat, where radical cleric Maulana Fazlullah waged armed rebellion to enforce strict Taliban-style laws and establish a parallel judicial system.

Pakistani security forces were deployed in Swat, previously popular tourist destination, last year to quell the revolt.

Months of fierce fighting have killed hundreds of militants, scores of security personnel and civilians. (dpa)