Pakistani officials say unidentified gunmen launched a fresh spate of attacks on polio workers on Wednesday, after six workers involved in United Nations-backed polio vaccination efforts were killed, earlier this week.

Authorities say one person was wounded after gunmen opened fire on a polio team in the northern city of Peshawar.

Officials say two other teams were targeted in the towns of Nowshera and Charsadda, but no one was injured.

On Tuesday, four female workers were killed in a series of coordinated attacks in the port city of Karachi, a day after a male volunteer working on the joint local government-World Health Organization project was shot dead in the city.

Another woman working in the eradication effort was shot dead Tuesday in Peshawar.

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

There have been no claims 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.

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

However, during a Tuesday meeting, Prime Minister Raja Pervez Ashraf voiced support for the polio campaigns, saying Pakistan could not allow polio to “wreak havoc” on the lives of children.

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

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