World health officials have suspended a polio vaccination campaign in Pakistan, after a series of attacks on polio workers left at least eight people dead.

The World Health Organization and UNICEF directed health workers to stop the nationwide vaccination program, on Wednesday, after unidentified gunmen opened fire on members of at least three more polio vaccination teams.

Police say a female health worker and her driver were shot and killed in the northern town of Charsadda.

Authorities say another person was shot and wounded near Peshawar and a third shooting took place in the northern town of Nowshera. No one was injured in the incident.

Earlier in the week, six people were killed and several others wounded during attacks in Peshawar and the port city of Karachi. The shootings have occurred during a three-day nationwide anti-polio drive.

It is unclear if Taliban militants have played a role in the attacks. The group has previously condemned polio campaigns as a cover for spying.

The Taliban began voicing opposition to the health program after a Pakistani doctor was imprisoned for helping U.S. intelligence agents run a fake hepatitis vaccination program aimed at locating then-fugitive al-Qaida chief Osama bin Laden. Some militants also claim the vaccine itself causes harm.

Investigators say most of the attacks appear to have been carried out by gunmen on motorbikes.

In July, armed men killed a Pakistani doctor working for the World Health Organization on a national immunization campaign.

Pakistan, Afghanistan and Nigeria are the only three countries were polio remains endemic.