Gunmen killed at least 10 people at two Nigerian polio clinics on Friday, with one of the wounded health workers describing a horrifying attack that saw them trapped by fire.
Nigerian President Goodluck Jonathan condemned the "dastardly terrorist attacks" and vowed to bring the perpetrators to book, his office said.
The attacks in the northern city of Kano came after a local cleric denounced polio vaccination campaigns this week and some local radio programmes repeated conspiracy theories about the campaigns being a Western plot to harm Muslims.
Such conspiracy theories have long spread in parts of Nigeria's mainly Muslim north, stoked by local politicians. Nigeria is one of only three countries still considered to have endemic polio, along with Pakistan and Afghanistan.
"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.
A doctor speaking on condition of anonymity later said that one of the wounded taken to hospital died from her injuries, raising the death toll to 10.
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."
A victim in the first attack who was shot in the back said from her hospital bed that two gunmen stormed into the consultation room at the clinic where she and five other immunisation workers were seated and opened fire, killing two people.
They then set fire to a curtain, and shut the door behind them as they fled.
"We summoned courage and broke the door because we realised they wanted to burn us alive," the woman said.
Speaking haltingly at Aminu Kano Teaching Hospital, she declined to give her name or age, though she looked to be in her 30s.
Three other wounded victims from the same attack were hospitalised, including one who was in emergency surgery. One later died.
Those killed in the first attack included a man and two women, while the seven killed in the other attack were all women, according to residents.
At the clinic where seven were killed, windows were broken, the front door was blackened by fire and blood could be seen on the floor.
The two clinics are about five kilometres (three miles) apart.
In a statement in Abuja President Jonathan sympathised with the families of the slain health workers and promised to "ensure that the mission to totally eradicate polio from Nigeria in which they were patriotically engaged is carried out to a very successful conclusion."
The statement said "the government will continue to do everything possible to track down and apprehend agents of terrorism in the country".
Last year, gunmen killed two Nigerian police guarding polio vaccination workers, though it was unclear whether the attack was linked to the campaign.
Islamist 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 investigated.
The suspension followed allegations by some Muslim clerics that the vaccine was laced with substances that could make girls infertile as part of a US-led Western plot to depopulate Africa.
Despite the resumption of immunisations, polio has not been eradicated in Kano as many parents still reject the vaccine.
However, conspiracy theories have not been the only reason for scepticism.
US pharmaceutical giant Pfizer carried out a highly controversial trial here in 1996 for a meningitis drug blamed for the deaths of 11 children and disabilities in dozens of others.
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 stamp out polio, including in Nigeria.
While pockets of resistance have remained, public outreach efforts have succeeded in many areas of Nigeria in convincing families of the need for polio vaccines.