Plainclothes police officers examine the site of a bomb explosion, in Kabal, an area of Pakistan's Swat Valley, on Monday. AP

Provincial police chief Akhtar Hayat said it was not clear yet what caused the explosions in northwestern Swat Valley.

He said there was an old ammunition store in the office, and the police were probing whether it caused the explosions or it was a militant attack. Hayat said most of those killed were police.

Senior police officer Attaullah Khan said an initial blast at the facility in the Swat Valley in Khyber Pakhtunkhwa province was followed by a larger, more intense one.

Security personnel gather at the site after explosions occurred inside a police station in Swat valley. AFP

No one immediately claimed responsibility for the attack but in recent months, the Pakistani Taliban have claimed similar attacks after ending a ceasefire with the government.

Attaullah said part of the building collapsed and rescue workers retrieved three bodies and the 30 wounded people. He said the death toll could increase.

The police complex also houses the Kabal district police station and headquarters of a reserve police force but the main damage was done at the counter-terrorism department building.

Agencies