Nine female polio vaccinators have been killed in two shootings at health centres in northern Nigeria, police have told the BBC.
In the first attack in Kano the polio vaccinators were shot dead by gunmen who drove up on a motor tricycle.
Thirty minutes later gunmen targeted a clinic outside Kano city as the vaccinators prepared to start work.
Some Nigerian Muslim leaders have previously opposed polio vaccinations, claiming they could cause infertility.
On Thursday, a controversial Islamic cleric spoke out against the polio vaccination campaign, telling people that new cases of polio were caused by contaminated medicine.
Such opposition is a major reason why Nigeria is one of just three countries where polio is still endemic.