A bomb exploded next to a van carrying Pakistani security guards tasked with protecting workers involved in an anti-polio drive in the country's northwest on Monday, killing two people, according to officials.

The attack was the latest incident of violence against the government- and U.N.-backed effort to eradicate polio from Pakistan.

The bomb killed a police officer and a member of a volunteer peace committee, said senior superintendent of police operations for Peshawar district, Najeeb ur-Rehman. Police initially reported that six people died but ur-Rehman said that figure was later revised to two.

The attack happened in the village of Malikhel, about 20 kilometers (12 miles) outside the provincial capital of Peshawar. The victims were supposed to be protecting workers administering anti-polio vaccine to local residents.

In 2011, Pakistan had 198 confirmed polio cases, the highest number of any nation in the world. It was able to bring that number down to 58 in 2012 through an aggressive vaccination program.

But the success has come at a steep cost.

Militants who oppose the vaccinations often target workers delivering the vaccine and threaten people who want to get their kids vaccinated.

Two powerful Pakistani Taliban militants have banned vaccinators from two tribal regions in the country's northwest, North and South Waziristan, over roughly the past year because of their opposition to U.S. drone strikes.

Militants claim the vaccine is meant to sterilize Muslim children and accuse health workers of being U.S. spies. The allegation gained traction after the CIA used a Pakistani doctor to try to confirm the presence of Osama bin Laden in 2011 under the guise of an immunization program.

Many suspect the Taliban of carrying out the murders, although the group has denied the allegation.