A suicide attack on a polio vaccination center in southwestern Pakistan on Wednesday killed 14 people, officials said.

The bombing of the polio center on the outskirts of Quetta city killed 12 police, a soldier and a civilian, said Syed Imtiaz Shah, the local police chief. Another 23 people were wounded, he said.

The suicide bomber detonated his explosives among the police officers, who were on patrol, said provincial home minister Sarfraz Bugti. “We’re in a war zone,” he said.

The bombing happened outside the polio center shortly before vaccination teams were due to be dispatched to local neighborhoods as part of a three-day immunization campaign, Shah said.

No one has claimed responsibility for the blast.

Polio workers in Pakistan, and their police escorts, have been the victims of several terrorist attacks in recent years.

Pakistani militants accuse polio workers of operating as spies for the United States. The Islamic militants intensified their attacks on vaccination teams after a Pakistani doctor, Shakil Afridi, was arrested on charges of running a fake hepatitis vaccination campaign in the northwestern city of Abbottabad as a cover for a CIA-backed effort to obtain DNA samples from a home where Osama bin Laden was later killed in a 2011 raid by U.S. Navy SEALs.

Pakistan is one of the three countries in the world where polio is endemic, and Taliban attacks have badly hampered vaccination campaigns. Some Pakistanis are also suspicious about the vaccination, fearing it will sterilize their children.

The bombing made the security forces the primary target, said police chief Shah, speaking at the scene, which was strewn with blood and debris, of the bombing as rescuers rushed the wounded to hospital.

Quetta is the capital of southwestern Baluchistan province, which has seen a yearslong insurgency by nationalist and separatist groups, who have been fighting for greater shares in the region’s natural mineral resources.

Al Qaeda and local Taliban-linked Sunni militant groups have also been operating in the region, and often attack minority Shiite Muslims, government and security officials. Tens of thousands have been killed in the insurgent attacks over the past decade.