Officials in Nigeria's northern Kano state say parents who refuse to have their children vaccinated against polio may be prosecuted.

The government order issued this week comes as the United Nations children's agency, UNICEF, has been pressuring Nigeria's northern states to promote vaccination against the highly contagious disease. Officials began a four-day immunization campaign in Kano on Thursday, with the goal of immunizing six million children.

Parents caught disobeying the order could face jail time.

The World Health Organization says a polio outbreak began spreading in the second half of 2008. Efforts to contain it have been complicated by rumors that the vaccine is harmful to children.