KARACHI, Pakistan – Gunmen shot dead five women working on U.N.-backed polio vaccination efforts in two different Pakistani cities on Tuesday, officials said, a major setback for a campaign that international health officials consider vital to contain the crippling disease but which Taliban insurgents say is a cover for espionage.

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 children sterile. Taliban 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 help the CIA track down al-Qaida chief Osama bin Laden, who was hiding in the town of Abbottabad in the country's northwest.