Relatives and rescue workers carry the coffin of a man killed in a bomb blast in Peshawar on Monday. Khuram Parvez/Reuters

A bomb hit a police van protecting a polio-vaccination team in northwestern Pakistan Monday, killing two people and wounding up to 20 others, police said. The incident is the latest in a series of attacks against U.N.-backed health workers, who have been targeted by armed groups that accuse them of serving as fronts for Western governments.

"It was an IED (improvised explosive device) blast, and the target was policemen," said Najeeb ur-Rehman, a police official in the city of Peshawar, where the blast occurred. Rehman said that one of the victims killed and most of the wounded were police officers. Other police reports stated that up to six people had died in the attack.

Pakistan — one of only three countries where polio is endemic — reported nearly 200 confirmed cases of the disease in 2011, the most for any country that year. Pakistan was able to bring that number down through a vaccination program that is supported by the U.N., but threats of violence could reverse its progress.

Polio, which can cause irreversible paralysis within hours of infection, is spreading rapidly throughout Pakistan. Eight new cases were reported in the country last week, according to the Global Eradication Initiative. In August local health officials detected five other cases.

Health workers had planned to immunize 34 million children across the country, but 1.5 million have not received the vaccine because of security threats, said Elias Durry, the World Health Organization's emergency coordinator for polio eradication in Pakistan.

Gunmen killed two female health workers in northwestern Pakistan earlier this year. Similar attacks have been staged elsewhere in the country and hampered efforts by global health organizations to eradicate the disease.

A small but vocal minority of religious leaders in Pakistan accuses the West of using vaccination programs as cover for a variety of anti-Islamic plots. The allegation gained traction after the CIA used a Pakistani doctor, under the cover of an immunization program, to try to confirm Osama bin Laden's whereabouts in 2011.

Violence in Pakistan's often lawless northwest has shown few signs of abating, despite efforts to open talks with Taliban groups in the country and in neighboring Afghanistan.

Threats by armed groups have also hindered vaccination efforts in Somalia, where polio outbreaks have flared up this year. The country has confirmed 108 cases so far, according to the Global Polio Eradication Initiative website — the worst epidemic in the world.

Al Jazeera and wire services