Gunmen have shot dead five health workers on an anti-polio drive in a string of attacks in Pakistan.

It was not clear who was behind the shootings but Taliban insurgents have repeatedly denounced the anti-polio campaign as a Western plot.

Health officials have suspended the immunisation campaign in Karachi, Pakistan's biggest city of 18 million people.

The move came after three women were killed and a man was wounded in two separate attacks.

In the northwestern city of Peshawar, gunmen on a motorbike shot dead a 17-year-old girl supervising an anti-polio campaign.

An anti-polio worker in Karachi was also killed yesterday, the United Nations said.

All of the victims were Pakistanis working with a UN-backed programme to eradicate polio, which attacks the nervous system and can cause permanent paralysis within hours of infection.

It has been eradicated in all but a handful of countries, but at least 35 children in Pakistan have been infected this year.

In Karachi, provincial Health Minister Saghir Ahmed said the government had told 24,000 polio workers it was suspending the anti-polio drive in the province.

Officials could not confirm if all the attacks were linked to the health campaign, Michael Coleman for the United Nations Children's Fund said.

Many of the attacks occurred in areas notorious for gun violence but the situation was a worry, he said.

There have been at least three other shootings involving polio eradication workers this year.

Some Islamists and Muslim preachers say the police vaccine is a Western plot to sterilise Muslims.

Other religious leaders have taken part in campaigns aimed at debunking that myth.