Syed Kamal Shah, North West Frontier Province (NWFP) health minister, told AFP news agency: "We are facing difficulty in the relief operation because the blast also damaged an electricity transformer in the area.
"We are still retrieving injured and dead from the blast site and taking them to hospital."
The hospital facilities could not cope and there were not enough emergency supplies due to the ongoing military operation against armed groups in the area, he said.
Battle ground
Pakistani troops have been battling Muslim fighters in Swat valley, a once popular tourist site.
Major-General Nasser Janjua, a regional commanding officer, said earlier in the week that 400 fighters were hiding in the valley.
"Nobody has claimed the responsibility for the attack, but we suspect the involvement of miscreants (militants) against whom the military operation was being carried out," another senior security official said.
Pakistani forces have been searching for Maulana Fazlullah, a religious leader who called for Islamic law in the valley. The army launched a major offensive in November to drive his followers out of Swat.
The mountainous, snow-capped Swat region is renowned for its ancient Buddhist relics and once attracted large numbers of foreign and local tourists, but has since been beset by violence.
Hundreds of people have died across northwest Pakistan in recent months.
A suicide bomb campaign targeting security forces intensified after the army stormed Islamabad's Red Mosque last July to crush a conservative student movement.
Last year, about 2,000 people were killed in violence across Pakistan.