Gunmen attacked two polio clinics in the northern Nigerian city of Kano on Friday, killing nine people before fleeing, police and residents said.
"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.
The attacks come after a local cleric denounced polio vaccination campaigns this week and some local radio programmes repeated previous conspiracy theories about such campaigns being a Western plot to harm Muslims.
Such conspiracy theories have long spread in parts of northern Nigeria, stoked by local politicians. Nigeria is one of only three countries still considered to have endemic polio, alongside Pakistan and Afghanistan.
Police declined to say who they believed was behind the killings. Islamist extremist group Boko Haram has carried out attacks in Kano, though gangs linked to local politics also operate.
"Six people on a tricycle pulled up outside the dispensary at 9:45 am while polio immunisation workers were gathering for the day's house-to-house polio campaign," a resident near the second attack said.
"Two of the men were holding guns. They stormed into the dispensary and began shooting."
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.
Islamists extremists have carried out scores of attacks on police.
In 2003, Kano's state government suspended polio immunisations for 13 months, with the then governor saying claims on its harmful effects had to be looked into.
The suspension followed allegations by some Muslim clerics that the vaccine was laced with substances that could render girls infertile as part of US-led Western plot to depopulate Africa.
Despite the resumption of polio immunisations, Kano has continued to record polio cases as many parents still reject the vaccine.
Deadly attacks linked to polio vaccination campaigns have also occurred in Pakistan.
On January 31, a bomb killed two polio vaccination workers in Pakistan's northwestern tribal region, the latest in a series of such attacks in the country which have killed 19 people in two months.
Polio once crippled and killed children worldwide until a concerted eradication campaign was started in 1988.
According to the World Health Organisation, Nigeria accounted for 121 of the world's 222 polio cases in 2012.
Microsoft co-founder Bill Gates and his foundation have led a high-profile effort to completely eradicate polio, including in Nigeria.
While pockets of resistance have remained, public outreach efforts have seen success in many areas of Nigeria in convincing families of the need for polio vaccines.