A suicide attack on a polio vaccination center in southwestern Pakistan on Wednesday killed 15 people, mainly policemen gathered to escort health workers, officials said. It was the latest attack on the vaccination campaign and health workers have been repeatedly targeted in recent years by Islamic militants.

The bombing took place on the outskirts of the city Quetta, killing 13 policemen, a soldier and a civilian while wounding 23 more.  The suicide bomber detonated his explosives among the police officers, just outside the polio center shortly before vaccination teams were due to be dispatched to local neighborhoods as part of a three-day immunization campaign.

Hours after the attack, Ahmad Marwat, who described himself as a spokesman for Jundullah, or Army of God, a little-known militant group, claimed responsibility for the assault, without explaining why the center was targeted. He warned of more attacks on polio teams in the future.

Polio workers in Pakistan, and their police escorts, have been targeted in recent years by Islamic militants who accuse them of working as spies for the United States.  The attacks intensified after Shakil Afridi, a Pakistani doctor was arrested on charges of running a fake hepatitis vaccination campaign in the city of Abbottabad as a cover for a CIA-backed effort to obtain DNA samples from children living in the compound of Osama bin Laden in an effort to confirm he was living there.  It was the same compound of the 2011 U.S. raid that killed him.

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

Pakistan recorded more than 300 polio cases in 2014 – its highest number since 1999. Most of the new infections were in north-west Pakistan, where militants regularly target roving health teams, and health officials blamed the rise in cases on several deadly attacks on police workers that year.

The number of cases fell to just over 50 in 2015, largely because vaccination teams could reach areas that were previously off limits because of militancy.  This is one of the bloodiest attacks targeting a polio vaccination campaign in the Balochistan region in recent months.  It comes in the wake of a sharp fall in militant attacks in Pakistan, mainly caused by a ground offensive the military launched in June 2014 to clear the North Waziristan region of Taliban militants. As such, it serves as a stark reminder that door-to-door polio vaccination continues to be a hazardous occupation.