Two workers in a polio eradication campaign were shot dead in Pakistan, the latest in a string of attacks against the UN-backed campaign.

The United Nations has pulled all staff involved in the immunisation campaign off the streets.

In the northwestern district of Charsadda, men on motorbikes shot dead a woman and her driver.

Hours earlier, a male health worker was shot and badly wounded in the nearby provincial capital of Peshawar.

Four other women health workers were shot at but not hit in nearby Nowshera, according to the deputy head of the Expanded Programme on Immunisation.

It is not clear who is behind the violence but some Islamists, including Taliban militants, have long opposed the campaign, with some saying it is aimed at sterilising Muslims.

Six women health workers were killed in attacks in the southern port city of Karachi and in Peshawar over the last two days.

The youngest was 17 years old.

The Taliban has repeatedly issued threats against the polio eradication campaign and health workers said they received calls telling them to stop working with the "infidels".

However, a spokesman for the Pakistani Taliban told Reuters his group was not involved in the violence.

The shootings, five of which happened in Karachi, home to 18 million people, led provincial health authorities to suspend the polio eradication campaign in the province of Sindh.

But authorities in Khyber Paktunkhwa province, where the capital is Peshawar, said they would not accept a recommendation to suspend the campaign even as the UN ordered its staff to suspend work.

The Taliban has repeatedly said the campaign is a Western conspiracy to sterilise or spy on Muslims or said the vaccinations could only continue if attacks by US drone aircraft stopped.

Its suspicions increased after it emerged that the CIA had used a fake vaccination campaign to try to gather information about Osama bin Laden, before he was found and killed in a northern Pakistani town last year.

Pakistan Prime Minister Raja Pervez Ashraf said the campaign needed to go on.

"We cannot and would not allow polio to wreak havoc on the lives of our children," he said in a statement.

Pakistan had 20,000 polio cases in 1994 but vigorous vaccination efforts had brought the number down to 56 in 2012, the statement said.

A global vaccination campaign has eradicated the disease from everywhere except Pakistan, Afghanistan and Nigeria.

Polio can paralyse or kill within hours of infection. It is transmitted person-to-person, meaning that as long as one child is infected, the disease can be passed to others.