Three North Korean doctors have been killed in a pre-dawn attack in Nigeria's northeastern town of Potiskum, police say.

The motive of Sunday's attack was not immediately clear, but a security source told Al Jazeera that preliminary reports suggested it was a robbery that resulted in the medics' deaths.

Yobe State police commissioner Sanusi Rufai initially said the victims were Chinese.

"Unknown attackers scaled over the fence of an apartment in Potiskum housing three Chinese doctors around 1am and slit their throats. No arrest has been made," he told the AFP news agency on Sunday.

Residents said the dead were employees of the state ministry of health and had been living in the volatile city for one year.

Nine health workers who were administering polio vaccinations were shot dead in the northern city of Kano on Friday.

Kano, Nigeria's second largest city, has been regularly targeted by Boko Haram, an armed group which has condemned the use of Western medicine.

The group is also active in Potiskum, where Sunday's killings happened, but there was no immediate claim of responsibility for the killings.

"It is still premature to point any accusing fingers but we have commenced investigation to unravel the killings," Rufai said.

Source: Agencies