Gunmen shot five health workers on an anti-polio drive in a string of attacks in Pakistan on Tuesday, officials said, raising fears for the safety of workers immunizing children against the crippling disease.
It was not clear who was behind the shootings but Taliban insurgents have repeatedly denounced the anti-polio campaign as a Western plot.
The immunization campaign has been suspended in Karachi, Pakistan's biggest city of 18 million people, health officials said.
Three women were killed and a man was wounded in two separate attacks on health workers in Karachi on Tuesday, said senior police superintendent police Imran Shaukat. An anti-polio worker in Karachi was shot dead on Monday, the United Nations said.
In the northwestern city of Peshawar, a woman supervising an anti-polio campaign there was wounded on Tuesday when gunmen on a motorbike shot her in the head, said government official Javed Marwar.
All of the victims were Pakistanis working with a U.N.-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, Health Minister Saghir Ahmed said the government had told 24,000 polio workers it was suspending the anti-polio drive in he 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.
Many of the attacks occurred in areas notorious for gun violence but the situation was deeply concerning, he said.
"We're concerned for the safety of front-line workers. They are the true heroes," 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 sterilize Muslims. Other religious leaders have taken part in campaigns aimed at debunking that myth.