ISLAMABAD, Pakistan — Gunmen shot dead five female health workers who were immunizing children against polio on Tuesday, causing the Pakistani government to suspend vaccinations in two cities and dealing a fresh setback to an eradication campaign dogged by Taliban resistance in a country that is one of the disease’s last global strongholds.

“It is a blow, no doubt,” said Shahnaz Wazir Ali, an adviser on polio to Prime Minister Raja Pervez Ashraf. “Never before have female health workers been targeted like this in Pakistan. Clearly there will have to be more and better arrangements for security.”

No group claimed responsibility for the attacks, but most suspicion focused on the Pakistani Taliban, which has previously blocked polio vaccinators and complained that the United States is using the program as a cover for espionage.

The killings were a serious reversal for the multibillion-dollar global polio immunization effort, which over the past quarter century has reduced the number of endemic countries from 120 to just three: Pakistan, Afghanistan and Nigeria.