Nine health workers who were administering polio vaccinations have been shot dead in two separate attacks in Nigeria's main northern city of Kano.

No one has claimed responsibility for the attack.

However, Islamist militant group Boko Haram 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 the polio vaccination, saying it is a conspiracy against Muslim children.

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.

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 sharia law 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 the country.

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 setback 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.