Gunmen killed at least 10 people in horrifying attacks on two Nigerian polio clinics, dealing a new blow to the campaign to wipe out the disease.
Seven women were shot dead at one clinic and a man and two women were killed at another. Both attacks were in the northern city of Kano.
Nigerian President Goodluck Jonathan condemned the "dastardly terrorist attacks" that came after a local cleric denounced polio vaccination campaigns and some local radio stations aired conspiracy theories about the vaccine being a Western plot to harm Muslims.
The United Nations and the United States also slammed the killings, with the world body saying the population was being "robbed of basic life-saving health interventions."
"Any violence that prevents children from receiving basic lifesaving vaccines is absolutely unacceptable, wherever it happens," US State Department spokeswoman Victoria Nuland told reporters.
Conspiracy theories against the polio campaign have long been spread in Nigeria's mainly Muslim north. Nigeria is one of only three countries -- with Pakistan and Afghanistan -- still considered to have endemic polio.
Police spokesman Magaji Majia told AFP that nine people were killed on the spot when gunmen on motorised tricycles "attacked two dispensaries where polio immunisation workers were preparing to go out for polio campaigns."
One woman died later in hospital from injuries suffered, raising the death toll to 10, a doctor said.
Police declined to say whether any group was suspected.
Extremist group Boko Haram has carried out attacks in Kano, though gangs linked to local politics also operate there.
The two clinics are about five kilometres (three miles) apart.
At the building where seven women were killed, windows were broken, the front door was blackened by fire and blood could be seen on the floor.
Six people on a big-wheel tricycle pulled up outside the second dispensary as polio immunisation workers were gathering for the day's house-to-house campaign, a resident said.
"Two of the men were holding guns. They stormed into the dispensary and began shooting," the person added.
One victim, who was shot in the back, said from her hospital bed that two gunmen stormed into the consultation room where she and five other immunisation workers were seated and opened fire, killing two people.
They then set a curtain ablaze 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," added the woman, who declined to give her name or age.
Three other victims wounded in the attack were hospitalised and one later died.
Jonathan expressed sympathy to the families of the dead health workers and vowed 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 government will continue to do everything possible to track down and apprehend agents of terrorism in the country," he added.
In a joint statement, the UN Children's Fund (UNICEF) and the World Health Organization condemned the killings as a "double tragedy: for the health workers and their families and for children and vulnerable populations who are robbed of basic life-saving health interventions."
Last year, gunmen killed two Nigerian police guarding polio vaccination workers.
In 2003, Kano's state government suspended polio immunisations for 13 months, with the governor saying claims of harmful effects had to be investigated.
Some Muslim clerics have alleged 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, many parents still reject the vaccine.
However, conspiracy theories have not been the only reason for scepticism.
US pharmaceutical giant Pfizer carried out a controversial trial in 1996 for a meningitis drug blamed for the deaths of 11 Nigerian 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 that have killed 19 people in two months.
A concerted campaign to eradicate the disease that once crippled and killed children worldwide was started in 1988.
According to the WHO, 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.