ISLAMABAD, Pakistan—A suspected Taliban suicide bomber killed at least 55 people in Pakistan's Mohmand tribal region Friday as they queued outside a government office, in a strike that local officials say was targeting pro-government tribal elders.

After an initial explosion, a suicide bomber on a motorbike drove into a crowd of people who had gathered in the morning outside a government building near Yakaghund, a town in Mohmand, one of seven semi-autonomous tribal agencies located near Pakistan's northwest border with Afghanistan.

More than 100 other people were injured in the attack, which also damaged a nearby market. Six children and several paramilitary soldiers were among the dead, officials said. The injured, some in a critical state, were taken to hospitals in the nearby town of Peshawar, the gateway to the tribal regions.

Many more people were still trapped in the rubble of buildings flattened in the explosion. The walls of a nearby prison were blown out, allowing about 30 petty criminals to escape, said Rasool Khan, a senior Mohmand government official. It's not clear if any of the tribal elders, who were to gather at the provincial government office, were killed.

"It was a high-intensity explosion which shook the entire area. I think huge amounts of explosive material were used in the blast," Mr. Khan said. The government ordered a curfew and blocked a major road running through the area, officials said.

Pakistan's army, aided by U.S. pilotless drone strikes, has been fighting a two-year war in the tribal areas, which have become a haven for Taliban and al Qaeda operatives in recent years and a staging post for attacks against U.S. forces in Afghanistan.

The army has made advances against militants in some tribal regions, most notably in South Waziristan. Around 2,000 Pakistani soldiers have died in the campaign in the past two years. More than a million people have been displaced by the fighting, and many civilians have also been killed.

Surviving militants have fled South Waziristan to Mohmand and Orakzai, another tribal area. The Pakistan air force killed about a dozen militants in attacks in Orakzai on Friday, officials said.

Despite heavy losses and being pushed back to mountain hideouts, the Taliban and its militant allies are still able to strike regularly at military and government targets in the tribal regions and other parts of Pakistan.

Last week, two suicide bombers attacked a popular Muslim shrine in the eastern city of Lahore, killing at least 40 people. That attack has sparked armed street protests from moderate Muslim groups over the failure of the government to protect them from militant attacks.

The death toll on Friday was one of the largest in the northwest of the country since an attack on a political rally near Peshawar in April killed more than 50 people. That attack targeted a political party that has supported the military's offensive in the tribal regions.