A pair of suicide bombers drove two vehicles filled with explosives into a security installation in northwestern Pakistan on Tuesday, killing at least 12 civilians, among them six children, police and rescue services said.

The blast brought down the roof of a nearby mosque soon after residents had broken their fast during the Muslim holy month of Ramadan in a market teeming with shoppers, a military official said on condition of anonymity.

Security forces foiled the militants' bid to enter the military facility after the blast, the official said, adding that six of them were killed in an exchange of fire.

There was no immediate claim of responsibility. Recent years have seen growing attacks by the Pakistani Taliban known as Tehreek-e-Taliban Pakistan (TTP) Islamist militant group on police and military in areas near the Afghan border.

Twelve people were killed and 30 wounded in Tuesday's attack, said Muhammad Nauman, a spokesperson for a nearby hospital, all of them civilians caught under collapsed buildings and walls.

The tally of children killed was revised down to six on Wednesday from seven in a hospital list the previous day, Nauman added.

Rescue services said they were searching for more casualties under the debris of collapsed buildings, as video images from the scene showed people sifting piles of bricks and clearing metal scaffolding.

"The evil ambitions of the enemies of Pakistan will never be allowed to succeed," Prime Minister Shehbaz Sharif said in a statement condemning the attack.

Ali Amin Gandapur, the chief minister of the border province of Khyber Pakhtunkhwa, where the blast happened, condemned the incident.

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