Gunmen have shot dead at least nine health workers who were administering polio vaccinations in the northern Nigerian city of Kano, police say.
The attacks took place on Friday morning in two different locations in Kano.
"Nine people were killed in two separate attacks by gunmen on [motorised] tricycles when they attacked two dispensaries where polio immunisation workers were preparing to go out for polio campaigns," police spokesman Magaji Majia told AFP news agency.
Al Jazeera's Yvonne Ndege, reporting from Lagos, said that at one centre two people were killed and at least four people were injured. She said that at least seven people were killed at the other centre.
No groups or individuals have calimed responsibility for the attacks, but our correspondent Ndege said that the feeling in the area is that there is some sort of opposition to the vaccination drive of babies being vaccinated against polio.
Kano, Nigeria's second largest city, has been regularly targeted by Boko Haram, an armed group which has condemned the use of Western medicine.
There are individuals and groups in the country that believe that vaccines, like the polio vaccination are some sort of ploy by Nigeria's government, who with foreign pharmeceutical companies, are trying to somehow reduce Nigeria's muslim population, said Ndege.
"This goes way back to an incident that took place way back in the 1990's when the US pharmarceutical giant Pzifer ran meningitus drugs test here. It went wrong and dozens of children were killed," said Ndege.
Nigeria remains one of the few countries left in the world where polio remains endemic.
Last year, two Nigerian police guarding polio vaccination workers were killed by gunmen, though it was unclear if
the attack was linked to the campaign.
According to the World Health Organisation, Nigeria accounted for 121 of the world's 222 polio cases in 2012.