In a brutal pre-dawn attack, unknown assailants killed three South Korean doctors, beheading one of them, in Nigeria’s volatile northern state of Yobe, police said today.

“They invaded the apartment of the doctors….and slit their throats,” Sanusi Rufai, the commissioner of police of Yobe state said on the attack that occurred in the town of Potiskum.

Residents say the slain doctors worked at a general hospital in the city.

Two of them had their throats slit while the third was beheaded, they added.

The victims were reportedly found inside their flat this morning, after people became worried that they were not answering the door.

Officials said the victims had been working at a government-run hospital.

No group has claimed responsibility for the attack.

Yobe shares border with Borno state where terrorist group Boko Haram has its headquarters.

Nigerian President Goodluck Jonathan condemned the killings, for which no group has claimed responsibility, and vowed that the campaign to eradicate polio would be carried through to a successful conclusion.

Last week, nine health workers on polio vaccination drive were shot dead by suspected Islamists in northern city of Kano.

Potiskum has witnessed repeated attacks by the group which has vowed to turn Nigeria into an Islamic state.

Boko Haram has a breakaway terrorist sect known as Ansaru which has attacked foreigners in the past.

More than 600 people were believed to have been killed in 2012 by the group, which is fighting to overthrow the government and create an Islamic state in Nigeria’s predominantly Muslim north.