Nine Nigerian health workers administering polio vaccinations have been shot dead by gunmen on motorbikes.

They were killed in two separate attacks in Nigeria's main northern city of Kano.

"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."

Military cordons have been set up around the areas.

Although the gunmen have not been identified, local reports claiming that Islamist militants in Boko Haram were responsible.

The extremist group, which has condemned the use of Western medicine in Nigeria, is believed to be behind a recent spate of attacks on security forces in Kano.

Boko Haram killed hundreds of people in 2012 as part of its campaign to impose strict sharia law on a country of 160 million, split almost equally between Christians and Muslims.

In 2003, northern Nigeria's Muslim leaders opposed polio vaccinations because they believed it could cause infertility and Aids. They said the vaccination programme was a conspiracy against Muslim children.

Polio, which attacks the nervous system and can cause irreversible paralysis within hours of infection, affects thousands of people in Kano every year.

Nigeria, Pakistan and Afghanistan are the only three countries still considered to have endemic polio,