Pakistani officials say gunmen attacked a polio vaccination team in the country's northwest, killing a policeman who was escorting the health workers.
The group was assaulted on May 20 in the Bajur tribal area, one of Pakistan's seven tribal areas along the Afghan border.
A local official said more than 600 teams are taking part in the latest campaign in Bajur to vaccinate 220,000 children against the crippling disease.
It is unclear whether the campaign, which had already been postponed due to previous attacks, will continue.
No one has claimed responsibility.
In December, gunmen killed at least nine polio workers in separate attacks in Pakistan.
Some Pakistani militants oppose the polio vaccination campaign, accusing health workers of acting as spies for the United States and claiming the vaccine harms Muslim children.
Based on reporting by AFP and dpa