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 also have said vaccinations can’t go forward until the U.S. stops drone strikes in the country.