Gunmen in Pakistan have shot dead six health workers associated with an anti-polio campaign in a string of attacks.

It was not clear who was behind the shootings in the southern city of Karachi and northern city of Peshawar on Tuesday, but Taliban fighters have repeatedly denounced the anti-polio campaign as a "Western plot".

The government's immunisation campaign against the crippling disease was suspended in Karachi following the attacks.

Five women were killed and two men wounded in two separate attacks on health workers in Karachi on Tuesday, according to Al Jazeera sources.

The team had received telephone calls warning workers they would regret helping the "infidel" campaign against polio, said Gul Naz, a health official who oversees the project in the area where the women were shot.

Senior police officer Shahid Hayat blamed "militants who issued a fatwa against polio vaccination in the past" for the  killings. He said one attack happened in the neighbourhood of Gulshan-e-Buner.

"They were fired upon by unidentified gunmen who rode away on motorcycles. Two women members suffered multiple gunshots and died on the spot," he said.

Authorities said the attacks were co-ordinated and occurred at the same time in different parts of the city.

Peshawar attacks

In Peshawar on Tuesday, gunmen on a motorcycle shot a 17-year-old woman supervising an anti-polio campaign, said Javed Marwar, a government official. She later died of her wounds.

"We're concerned for the safety of front-line workers. They are the true heroes."

- Matthew Coleman, UNICEF

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

The disease has been wiped out in all but a handful of countries. At least 35 children in Pakistan have been infected this year.

In Karachi, Sagheer Ahmed, the provincial health minister, said that 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, said Matthew Coleman, a spokesman for the United Nations Children's Fund.

"We're concerned for the safety of front-line workers. They are the true heroes," he said.

Government campaign

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

Speaking to Al Jazeera from Islamabad, Zafar Jaspal, a professor at Quaid-e-Azam University, said in big cities like Karachi the health workers become "very easy targets", making it much more difficult for the government to protect them from threats.

The government, in conjunction with UN agencies, is on a nationwide campaign to give oral polio drops to 34 million children under the age of five.

In a joint statement condemning the killings, the World Health Organisation and UNICEF said such attacks "deprive Pakistan’s most vulnerable populations – especially children - of basic life-saving health interventions".

Vaccination programmes, especially those with international links, however, have been branded "conspiracies" by many religious leaders for years.

Matters were not helped when a Pakistani doctor ran a fake vaccination programme last year to help the CIA track down al-Qaeda leader Osama bin Laden.

Jaspal said some groups suspect the polio campaigns "have been exploited or misused for data collection" in areas with suspected Taliban presence.

Launching the polio drive on Monday, Pakistani authorities threatened to punish tribesmen in the country's tribal areas who refuse to allow their children to be inoculated.

Siraj Ahmad Khan, the top official in the North Waziristan tribal area, said the punishments would include a ban on monthly stipends to tribal elders, development work, civil service recruitment and issuing ID cards and passports.

Source: Al Jazeera and agencies