The attack took place in the restive Khyber tribal region, which straddles the border with Afghanistan and has been the site of a recent offensive by the Pakistani military. There was no immediate claim of responsibility for the attack, but polio vaccination workers are frequently singled out across Pakistan by Taliban militants, who view the campaign as un-Islamic and the health workers as Western spies.

Separately on Saturday, two polio workers along with two security guards were reported to have gone missing in the southwestern province of Baluchistan, officials said. It was not immediately clear whether they had come under attack or had merely fallen out of contact in the remote area where they were working.