Gunmen in northwest Pakistan Saturday killed at least 11 security officers escorting a team of health workers who came to vaccinate children against polio.
The attack was carried out in the Khyber Pakhtunkhwa region, near the border with Afghanistan.
It is unclear who was behind the attack.
Provinicial Governor Shaukatullah Khan said the health workers came to administer life-saving drops to children and that the killings violate local traditions.
Some religious leaders accuse health workers of being spies and say the medicines are meant to sterilize Muslim children.
Pakistan, Afghanistan, and Nigeria are the only three countries left where polio is still endemic.