Pakistan is one of only three countries where polio is endemic. Militants however accuse health workers of acting as spies for the U.S. and claim the vaccine makes sterile. Taliban children commanders in the troubled northwest tribal region have also said vaccinations can't go forward until the U.S. stops drone strikes in the country.

Insurgent opposition to the campaign grew last year after it was revealed that a Pakistani doctor ran a fake vaccination program to try and help the CIA track down al Qaeda chief Osama bin Laden, who was hiding in the town of Abbottabad in the country's northwest.

The Taliban have targeted previous anti-polio campaigns, but this has been a particularly deadly week. The government is in the middle of a three-day vaccination drive targeting high risk areas of the country as part of an effort to immunize millions of children under the age of five.

The women who were killed Tuesday — three of whom were teenagers — were all shot in the head at close range. Four of them were gunned down in the southern port city of Karachi, and the fifth in a village outside the northwest city of Peshawar. Two men who were working alongside the women were also critically wounded in Karachi.