Four women were killed in less than an hour in seemingly coordinated attacks in Karachi on Tuesday, a day after a man working on a local government-World Health Organization (WHO) project was also shot dead in the city, police said.
A sixth worker, also a woman, was killed on Tuesday in the northwestern city of Peshawar, which lies close to the tribal areas, a haven for the Taliban and other militants who ordered a ban on polio vaccinations in June.
Pakistan is one of only three countries where polio remains endemic, along with Afghanistan and Nigeria, but efforts to tackle the highly infectious crippling disease have been hampered over the years by suspicion.
The Taliban banned immunisations in the tribal region of Waziristan, condemning the campaign as a cover for espionage after the jailing of a doctor who helped the CIA find Osama bin Laden using a hepatitis vaccination programme.
Sagheer Ahmed, the health minister for Sindh province, of which Karachi is capital, said he had ordered a halt to the anti-polio drive in the city in the wake of Tuesday's shootings.
Senior police officer Shahid Hayat blamed "militants who issued a fatwa against polio vaccination in the past" for the Karachi killings.
He said one polio vaccination team was attacked in the eastern Karachi 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.
Two women and a man were wounded in two separate incidents in the city's west, Hayat said, adding that all of the victims were part of WHO-supervised polio vaccination teams.
In a Peshawar suburb, two attackers on a motorbike fired on two sisters working on vaccination, killing one, senior police official Javed Khan told AFP.
In Waziristan, a hub for Islamist militants, the Taliban ban -- to protest against US drone strikes and because they allege that the polio campaign is a cover for espionage -- risks the health of 240,000 children, officials say.
Launching the polio drive on Monday, Pakistani authorities threatened to punish tribesmen who refuse to allow their children to be inoculated.
Siraj Ahmad Khan, the top official in 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.
Tuesday's killings in Karachi took place in parts of the city dominated by Pashtuns, Hayat said. Pashtuns are the dominant ethnic group in northwest Pakistan and have a sizeable migrant population in Karachi.
WHO, a partner in government efforts to eradicate the disease, suspended vaccination activities in part of Pakistan's largest city in July after a spate of shootings.
A UN doctor from Ghana working on polio eradication and his driver were shot in Karachi and three days later a local community worker who was part of the same campaign was shot dead in the same area.