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PESHAWAR, Pakistan — A bomb blast at a polio center in restive southwest Pakistan killed 14 and injured 25 on Wednesday, according to police.

Twelve of those killed in the city of Quetta were police officers, local police chief Rab Nawaz Khan said. Nine of the 25 injured were listed in critical condition, he added.

The bomb exploded outside the center shortly before vaccination teams were scheduled to travel to neighborhoods as part of a three-day immunization campaign, another local police chief Syed Imtiaz Shah told Reuters.

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"Today was third day of the campaign and the police were supposed to provide security to the polio teams," Khan told NBC News. Officials were still figuring out whether the blast had been caused by a roadside bomb or a suicide bomber.

Related: 471 Parents Arrested for Refusing to Vaccinate Kids

Dozens of people have been assassinated in the militants' drive to prevent vaccinations and health teams in parts of the country travel with armed guards.

Pakistan is one of the last countries where polio, which has been eradicated in the West, is still endemic. The Taliban and some clerics in conservative and lawless strongholds in the northwest of the country have come out against the vaccinations, accusing polio workers of being spies for the U.S. and claiming they are part of a Western plot to sterilize children.

Related:Save The Children Expelled From Pakistan In Wake of Bin Laden Case

Attacks on vaccination teams grew after Pakistani doctor Shakil Afridi was arrested on charges of running a fake hepatitis vaccination campaign in Abbottabad as a cover for a CIA-backed effort to obtain DNA samples from a home where Osama bin Laden was later killed by U.S. Navy SEALs.