Two explosions have rocked a counter-terrorism office in northwest Pakistan, killing at least 12 people, police said, with over 50 injured. 

Provincial police chief Akhtar Hayat said it was not clear yet what caused Monday's explosions in northwestern Swat Valley, which was previously long controlled by militants before they were flushed out in military operations in 2007 and 2009. 

Hayat said there was an old ammunition store in the office, and police were probing whether that caused the explosions or if it was a militant attack. 

No one has so far claimed responsibility but in recent months, the Pakistani Taliban have claimed similar attacks after ending a cease-fire with the government. 

Hayat said most of those killed were police counter-terrorism officers. 

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'Death toll could increase'

The regional hospital administration said it received several wounded people, some of them in critical condition. 

Senior police officer Ataullah Khan said an initial blast at the facility was followed by a larger, more intense one.  Khan said part of the building collapsed and rescue workers retrieved bodies and rescued the wounded. 

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. 

The Pakistani Taliban, formally known as Tehrik-i-Taliban Pakistan, is a separate group from Afghan Taliban but linked to them.

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