A suicide bomber detonated explosives outside a polio eradication centre in Pakistan's western city of Quetta, killing more than a dozen people in the latest deadly assault on the campaign to fight the disease in the country.

Al Jazeera's Kamal Hyder, reporting from Islamabad, said most of the dead on Wednesday were security forces on their way to guard the vaccination centre. "One passer-by was also killed in the explosion," Hyder reported.

Two armed groups - the Pakistani Taliban and Jundullah, which has links to the Taliban and has pledged allegiance to ISIL - separately claimed responsibility for the attack.

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At least 14 people were killed and 20 others wounded in the blast, officials said.

"The security officials were in a vehicle outside the centre when the bomber detonated his explosive vest," Balochistan's Home Minister Sarfraz Bugti told journalists. 

"Health workers, police guards, and personnel from the paramilitary Frontier Corps force were preparing for a door-to-door polio vaccination drive as part of a three-day campaign when they were attacked." 

Medical teams brought the wounded to Civil Hospital in Quetta, capital of restive Balochistan province.

"I saw the suicide bomber running towards us, firing at us and then blowing himself up," Samiullah, a security officer and one of the wounded, told Al Jazeera. 

"Such attacks aiming to stop anti-polio campaigns have happened in the past as well, which have resulted in the deaths of many innocent people. We are trying our best to stop it."

Teams in Pakistan working to immunise children against the virus are often targeted by the Taliban and other groups, who say the campaign is a cover for Western spies, or accuse workers of distributing drugs designed to sterilise children.

The latest attack comes after the campaign against polio was relaunched in Quetta and other districts of Balochistan on Monday.

Pakistan and neighbouring Afghanistan are the only two countries in the world that remain on the World Health Organization's list of polio-endemic countries.

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Source: Al Jazeera and agencies