(Updates with death toll, cause of blast)

By Izaz Mohmand

PESHAWAR, Pakistan, Dec 28 (Reuters) - At least 30 people were killed in a suicide car bomb blast at a polling station in northwestern Pakistan on Sunday, during a by-election for a provincial assembly seat, police said.

"The death toll has reached to 30. It could rise further," Behraman Khan, head of the police station near the Buner town, where the blast took place, told Reuters by telephone. "It was a suicide attack."

Khan said the attacker, who was apparently alone, is believed to have driven the car, parked it in front of the school where the polling station was set up and detonated the explosives while polling was underway.

The attack is the latest in a string of blasts in a region where security forces are battling al Qaeda and Taliban-linked militants who have unleashed a wave of suicide and bomb attacks and target killings in response to operations against them.

Another police official said four children were among the dead and their bodies were mutilated beyond recognition. Khan said around a dozen people were wounded.

The school building where the polling station was set up collapsed after the blast.

The incident took place near Buner, a remote mountainous town in North West Frontier Province and near the Swat Valley where security forces have been fighting militants since last year.

Suspected militants executed three men on suspicion of spying for Pakistani forces and U.S. forces in Afghanistan in the North Waziristan region on the Afghan border, intelligence officials and residents said on Sunday.

Bodies of two of the victims were strung up from a bridge near the town of Mir Ali while the bullet-riddled body of the third man was found near the region's main town, Miranshah.

In Swat, about 34 militants and two soldiers were killed in clashes on Saturday, military officials said. There was no independent verification of the casualty estimate. (Additional reporting by Zeeshan Haider; Editing by Valerie Lee) ((zeeshan.haider@thomsonreuters.com; +92 51 281 0017; Reuters Messaging: zeeshan.haider.reuters.com@reuters.net))

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