MAIDUGURI - Attackers slit the throats of three doctors from the Democratic People's Republic of Korea (DPRK), killing them in the apartment they shared in northeastern Nigeria, police said on Sunday.

The unknown attackers ransacked the apartment in Potiskum, Yobe state, after killing the doctors in the early hours.

It was unclear who carried out the killings. Potiskum has been frequently targeted by Islamist sect Boko Haram, a group that killed hundreds last year in its effort to carve out an Islamic state in Nigeria's mostly Muslim north.

However, there could also be a criminal motive as foreign workers are associated with wealth in Nigeria. Western embassies have designated northeastern states "red zones" and advise their nationals not to travel there.

Boko Haram's insurgency has contributed to a breakdown in order across the north of Africa's most populous nation and security sources believe criminal gangs carry out deadly armed robberies hoping the militants will be blamed.

"The three North Koreans were slaughtered at about 1 am. They worked at the General Hospital," Yobe Police Commissioner Sanusi Rufai said over telephone interview.

The commissioner said in 2009 the country had struck an agreement with Yobe to send 18 doctors to different parts of the state.

A military source had earlier said that he believed those killed were Chinese or South Korean.

Four Chinese construction workers were shot dead by gunmen in two separate attacks in October and November last year in Borno state, which neighbours Yobe.

Gunmen on motorbikes shot dead nine health workers who were administering polio vaccinations in two separate attacks in Nigeria's main northern city, Kano, on Friday. Boko Haram condemns the use of Western medicine.

Boko Haram is seen as the most serious threat to the stability of Africa's top energy producer, and Western governments fear the country could become a base for operations of al Qaeda-linked Islamist groups in the Sahara.

President Goodluck Jonathan has highlighted links between Boko Haram and Saharan Islamists and said that relationship justified his decision to join efforts by French and West African forces to fight militants in Mali last month.