Witnesses in northern Nigeria's largest city say gunmen suspected to belong to a radical Islamic sect have opened fire on polio vaccine drives in the city, killing at least 12 people.

The attacks on two different locations happened Friday morning in Kano. Witnesses said the gunmen arrived on three-wheel taxis and opened fire, killing polio vaccine drive workers and others.

The witnesses spoke on condition of anonymity out of fear of angering the radical sect known as Boko Haram.

Officials with Nigeria's police and military did not immediately respond to requests for comment.

Nigeria remains one of the few countries left in the world where polio remains endemic. Several years ago religious clerics had some of their followers stop receiving the vaccines saying that it would sterilize their children.