Military spokesman Lt Col Sagir Musa also warned that the Boko Haram terrorist network plans more such killings and "massive attacks" aimed at the northeastern city where their extremist movement was born, Maiduguri, capital of Borno state.
"The Joint Task Force wishes to alert citizens ... Of the impending plans by Boko Haram terrorists to carry out massive attacks and bombings in Maiduguri metropolis and other parts of Borno State any moment from now and during the Eid al-Fitr."
Musa said Tuesday's attack in Bui town, 185 kilometres south of Maiduguri, killed eight civilians and appeared to be the beginning of the planned campaign.
Witness Bala Anjikui said the militants struck at night and targeted teachers, like him, and Islamic clerics.
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Indicating a change of tactics, he said the militants used only knives to kill their victims, apparently to keep the noise of gunshots from attracting soldiers.
Nigeria declared a state of emergency on May 14 to fight an Islamic uprising by militants in the northeast who want to impose Islamic law across a country divided almost equally between the predominantly Muslim north and mainly Christian south.
Earlier today, the military congratulated themselves on the arrest of a Nigerian terrorist leader in neighbouring Niger, saying it had foiled a plot to regroup and launch more attacks.
A statement today said intelligence agents from both countries had been on the trail of the leader who was arrested Saturday in a border area. It did not identify the leader or his group.
Northeast Nigeria borders Niger and Chad and troops from all three countries have formed a joint force to try to secure hundreds of kilometres of porous borders.