ISLAMABAD, July 15 (Reuters) - At least 12 Pakistani security forces were killed in a series of bomb blasts and an exchange of fire with Islamist militants on Sunday, the second major attack in the northwest in 24 hours, officials said.

The blasts hit as a convoy of police and paramilitary troops passed through the town of Matta near the hilly Swat area, well known as a stronghold of pro-Taliban militants.

"The death toll could go higher," an intelligence official told Reuters.

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