POTISKUM, Nigeria (AP) — Assailants in northeastern Nigeria killed three North Korean doctors, beheading one of the physicians, in the latest attack on health workers in a nation under assault by a radical Islamic sect, officials said Sunday.

The deaths Saturday night of the doctors in Potiskum, a town in Yobe state long under attack by the sect known as Boko Haram, comes after gunmen killed at least nine women administering polio vaccines in Kano, the major city of Nigeria's predominantly Muslim north.

The two attacks raise new questions over whether the extremist sect, targeted by Nigeria's police and military, has picked a new soft target in its guerrilla campaign of shootings and bombings across the nation.