A suicide bomber killed at least 15 people, most of them police, outside a polio eradication center in Pakistan's western city of Quetta on Wednesday, police said following the latest militant attack on the anti-polio campaign in the country.
The bomb ripped through a police van that had just arrived at the center to provide an escort for vaccination workers engaged in a drive to immunize all children under five years old in the underdeveloped western province of Balochistan.
"It was a suicide blast, we have gathered evidence from the scene," Ahsan Mehboob, the provincial police chief told Reuters. "The police team had arrived to escort teams for the polio campaign."
No one has claimed responsibility for the blast.
Teams in Pakistan working to immunize children against the virus are often targeted by Taliban and other militant groups, who say the campaign is a cover for Western spies, or accuse workers of distributing vaccines designed to sterilize children.
The latest attack killed at least 12 policemen, one paramilitary officer and two civilians, and wounded 25 others, officials said. They estimated the bomb contained about five kilograms (11 lb) of explosives.
Pakistan and neighboring Afghanistan are the only two countries in the world where polio remains endemic, the World Health Organization says.
The campaign to eradicate the virus in Pakistan has had some recent success, with new cases down last year, but violence against vaccination workers has slowed the effort.