At least 12 people have been killed after a suicide bomber blew himself up at a police station in Pakistan's northwest, officials say.

The attack in Mingora, the main city in the Swat valley district, on Sunday was largest since the military declared success in an offensive aimed at pushing Taliban fighters from the area.

"The policemen were being given training in Mingora town when a suicide bomber entered the ground and blew himself up near the recruits," Qazi Ghulam Farooq, the Swat police chief, told the AFP news agency.

Mian Iftikhar Hussain, the information minister in the North West Frontier Province government, said that the victims were members of a new community police force.

Al Jazeera's Kamal Hyder, reporting from the capital, Islamabad, said that the provincial government had called for new recruits to volunteer because so many officers had deserted due to the violence in the region.

"These volunteers were under training. We are told that between 60 and 70 were at the location when a suicide bomber penetrated into the area," he said.

"There are suspicions that he may have been one of the volunteer police himself, but we are not able to confirm that."

There was no immediate claim of responsibility.

Curfew imposed

Mohammad Idrees, a local police official, said that a curfew had been imposed in Mingora, with troops and police patrolling the city as people quickly shut their businesses in fear of more bombings.

Television footage showed officers gathering up mutilated bodies outside the police station, which had been bombed twice before in recent months.

Yusuf Raza Gilani, the Pakistani prime minister, condemned the killings and vowed to continue the battle against Taliban fighters.

"We will not allow the enemies of the country to succeed in their evil designs," a statement from Gilani's office quoted him as saying.

The attack came a day after the military announced that it had destroyed a camp near the town of Charbagh, where it said that suicide bombers were being trained to hit targets across the Swat valley.

The military is winding down a three-month offensive in the Swat valley and the surrounding areas.

Islamabad has claimed success in the battle and the two million people displaced by the fighting have largely returned, but sporadic clashes continue to take place.