Last week, President Goodluck Jonathan declared a state of emergency in three north-eastern states - Borno, Adamawa, and Yobe - after a series of deadly attacks by militant groups.

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Yobe state police commissioner Sanusi Rufai said it was nevertheless too early to know who was behind the attack, which happened either late on Saturday or before dawn on Sunday.

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The BBC's Tomi Oladipo in Lagos says President Jonathan wants to use his trip to the states of Borno and Yobe to assert his authority in the face of the growth security threat.

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There were mixed reports initially on where the victims were from, but the police chief of Yobe told CNN they were North Korean physicians assisting at Potiskum General Hospital as part of a government agreement.

CNN: STORY HIGHLIGHTS

In July and August 2012 suicide bombers tried but failed to kill the shehu - or traditional ruler - of the Borno emirate in north-east Nigeria as well as the emir of Fika in Yobe state.

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