ISLAMABAD, July 4 (Reuters) - Four Pakistani passersby were killed and a senior police official and his guard wounded in a bomb blast in the northwest on Wednesday, the second attack on security forces of the day, police said.

"The target was the DPO (District Police Officer). Our initial investigation suggests it was a time bomb," said a police official in the Swat region, where the attack took place.

Another senior police official said three civilians were killed.

Earlier on Wednesday, a suicide bomber rammed his vehicle into a military convoy in the northwestern town of Bannu, killing himself along with six soldiers and two children, security officials said.

It was not known if the attacks were linked to clashes between security forces and militant Islamic students at a radical mosque in Islamabad.

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