* Health workers administering polio vaccines
* Islamists Boko Haram behind recent attacks
* Nigeria is one of 3 countries where polio is endemic
Gunmen on motorbikes shot dead nine health workers who were administering polio vaccinations in two separate attacks in Nigeria's main northern city of Kano on Friday, police said.
No one claimed responsibility but Islamist militant group Boko Haram, a sect which has condemned the use of Western medicine, has been blamed for carrying out a spate of assaults on security forces in the city in recent weeks.
Some influential Muslim leaders in Kano openly oppose polio vaccination, saying it is a conspiracy against Muslim children.
The attacks will hit efforts by global health organisations to clear Nigeria's mostly-Muslim north of polio; a virus that can cause irreversible paralysis within hours of infection.
It is the second time this year that polio workers have come under attack by Islamist militants after gunmen killed aid workers tackling the disease in Pakistan last month.
"Gunmen on bikes opened fire on a health centre in the Hotoro district killing seven, while an attack on Zaria Road area of the city claimed two lives," said police spokesman Magaji Musa.
"They were working for the state government giving out polio vaccinations at the time of the attack," Musa added.
Kano government banned motorbikes from carrying passengers last month after the Emir of Kano, one of the country's most prominent leaders, was nearly killed when gunmen attacked his convoy, killing four of his aides.
Kano residents said soldiers had cordoned off the areas attacked and movement was being restricted in the city.
Boko Haram killed hundreds last year as part of its campaign to impose Islamic law, or sharia, on a country of 160 million split roughly equally between Christians and Muslims.
The group is seen as the most serious threat to the stability of Africa's top energy producer, and Western governments fear the country could become a base for operations of al Qaeda-linked Islamist groups in the Sahara.
President Goodluck Jonathan has highlighted links between Boko Haram and Saharan Islamists and said that relationship justified his decision to join efforts by French and West African forces to fight militants in Mali last month.
In 2003, northern Nigeria's Muslim leaders opposed polio vaccinations, saying they could cause infertility and AIDS.
Their campaign against the treatments was blamed for a resurgence of the disease in parts of Nigeria and other African countries previously declared polio-free.
Polio, a virus that attacks the nervous system, crippled thousands of people every year in rich nations until the 1950s. As a result of vaccination, it is now only endemic in three countries - Afghanistan, Nigeria and Pakistan.
According to the Global Polio Eradication Initiative, there were 121 new cases of polio in Nigeria last year, compared to 58 in Pakistan and 37 in Afghanistan.
"This is certainly a set back for polio eradication in Nigeria, but not a stop," said Oyewale Tomori, a campaigner for polio eradication in Nigeria.
"The best we can do is to work harder and see the end of polio ... so their loss will not end as a useless sacrifice."
At least 16 health workers taking part in polio vaccination drives were killed in attacks in Pakistan in December and January.
Local Taliban militants said they did not carry out those attacks although its leaders have repeatedly denounced the vaccination programme as a plot to sterilise people or spy on Muslims.