Pakistani officials say unidentified gunmen have killed six workers involved in United Nations-backed polio vaccination efforts in two different cities.

Police say four female workers were killed in a series of coordinated attacks in the port city of Karachi Tuesday, a day after a male volunteer working on the joint local government-World Health Organization project was shot dead in the city. Another women working in the eradication efforts was shot dead Tuesday in the northwestern city of Peshawar.

Investigators say most of the attacks appear to have been carried out by gunmen on motorbikes. Several other health workers were wounded in the incidents.

The health minister of Sindhi province stopped the anti-polio drive in the Karachi region Tuesday, as a result of the shootings.

There was no immediate claim of responsibility, but the attacks have taken place during a three-day nationwide anti-polio drive. The Taliban has condemned previous polio campaigns as a cover for spying.

The militant group 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.

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