Authorities say 21 people have been killed in violence across north and central Nigeria as religious and ethnic tensions continue to simmer in the region.

In Plateau state, where rioting has killed hundreds in recent years, police and activists say at least 17 people were killed in separate attacks overnight Tuesday. The spark of the violence remains unclear, though an advocate says at least 14 of the dead belonged to a Christian ethnic group there.

In Kano, police say two officers involved in guarding a polio immunization drive were shot and killed Tuesday. On Wednesday morning, authorities say gunmen shot dead two officials from the Federal Road Safety Corps, a national traffic agency.

The violence comes as Nigeria remains under guerrilla attack by a radical Islamist sect.