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.

The attackers apparently struck at the North Korean doctors inside their home, said Dr. Mohammed Mamman, chairman of the Hospital Managing Board of Yobe State. The North Korean doctors had no security guards at their residence and typically traveled around the city via three-wheel taxis without a police escort, officials said.

By the time soldiers arrived at the house, they found the doctors' wives cowering in a flower bed outside their home. At the property, they found the corpses of the men, all bearing what appeared to be machete wounds.

An AP journalist later saw the North Korean doctors' corpses before they were moved to nearby Bauchi state for safe keeping. Two of the men had their throats slit. Attackers beheaded the other doctor.

The doctors lived in a quiet neighborhood filled with other modest homes in the town.

Initially, doctors at the hospital who worked with the physicians identified them as being from South Korea, while police identified the dead as being from China. Ultimately, Mamman of the health board told journalists those killed were from North Korea and had lived in the state since 2005 as part of a medical program between the state and the North Korean government.

Yobe state police commissioner Sanusi Rufai confirmed the attack took place and said officers had begun an investigation. Rufai said officers had made some arrests after the killings, though police in Nigeria routinely round up those living around the site of a crime, whether or not there is any evidence suggesting their complicity.

No one claimed responsibility for the attack, though suspicion fell on the Boko Haram sect.

The killings of the doctors come after the attack Friday on polio vaccinators in Kano, northern Nigeria's most populous city. No group has yet claimed responsibility for that attack either, though it follows alleged Boko Haram attacks now focusing on softer targets, like lightly guarded mobile phone towers. Those mobile phone tower attacks have limited the ability of residents and security forces to call for help during attacks, as well as have cut the government's ability to use the signals to track suspected militants.