The World Health Organization and UNICEF condemned Tuesday the murder of six health care workers in Pakistan who were involved in a polio vaccination campaign.

At least five of the victims killed in a string of coordinated attacks in Karachi and Peshawar were women.

In response to the killings, the Pakistan government has temporarily halted the vaccination program in Karachi, the largest city.

“Such attacks deprive Pakistan’s most vulnerable populations – especially children – of basic life-saving health interventions,” WHO and UNICEF stated. “We call on the leaders of the affected communities and everyone concerned to do their utmost to protect health workers and create a secure environment so that we can meet the health needs of the children of Pakistan.”

Prime Minister Raja Pervez Ashraf also condemned the attacks and defended the work done by U.N.-backed vaccination teams to eradicate polio. Ashraf vowed to provide more protection for health care workers.

“We cannot, and will not allow polio to wreak havoc with the lives of our children,” Ashraf said in Islamabad, according to Pakistani media. “This is the least we owe to our children and we will stay the course until polio is wiped out from the country.”

Under the canceled program, Pakistani health officials planned to administer millions of “polio drops” to immunize people. The program involved 25,000 workers targeting more than 30 million children.

While no group has yet claimed responsibility for the murders, the Pakistan Taliban have long threatened anti-polio workers.

Opposition to such programs stems from a phony CIA hepatitis vaccination scheme that was apparently a ruse to find the whereabouts of Osama Bin Laden in 2011. Other health care workers and foreign NGO staff have been abducted and killed by militants in Pakistan over the years due to concerns they were “spies.”

Maulana Fazlullah, a Taliban leader in the northwest part of Pakistan, used radio addresses to warn that Western vaccine programs were designed to sterilize Pakistani children in order to control the Muslim population.

Polio, a highly infectious disease caused by a virus that can lead to permanent paralysis, remains endemic in only three countries: Afghanistan, Nigeria and Pakistan.

Last year, about 200 children were paralyzed by the disease, the highest number since 1996, BBC reported, with a low of only 28 cases reported in 2005.