Nine polio vaccination team members in northern Nigeria have been killed by unknown gunmen.
Some were killed in the city of Kano and others at a health centre outside the town. The BBC reports the two episodes took place 30 minutes apart.
Some Muslim leaders have previously opposed polio vaccinations, claiming they could cause infertility.
On Thursday, an Islamic cleric spoke out against the vaccination campaign, telling people that new cases of polio were caused by contaminated medicine.
Analysts believe the attacks may have been the work of the Boko Haram group - whose name translates as "Western education is forbidden". It is blamed for the deaths of some 1400 people in central and northern Nigeria since 2010.