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.