The bomber set off his explosives as he pulled up at a checkpoint in Charsadda, a town near the city of Peshawar.

There has so far been no claim of responsibility for the attack.

But correspondents say Pakistani Taleban militants, allied to al-Qaeda, have carried out numerous such attacks over the past two years.

The bombing shattered windows of buildings near the explosion and destroyed power cables, plunging the area into darkness, police told the BBC.

"We had information that some people from Swat wanted to get into Peshawar to carry out terrorist attacks, so we had beefed up the force at the check post," Peshawar police chief Sifwat Ghyur said.

He said that the bomb left a crater about 3m wide, while the surrounding area is littered with blood spattered bodies and debris.

The Charsadda district is near the Malakand division - including the picturesque Swat valley - where the government on Monday signed an agreement to enforce Sharia law as part of its efforts to end the long-running insurgency in the north-west.

The district has been hit by several suicide bombings and is home to the chief of the ruling Awami National Party, Asfandyar Wali Khan, who himself narrowly avoided a suicide attack last year.