A former local government official, a civilian militia member and a motorist who witnessed the shooting yesterday said the Islamist militants tried to storm the outpost in the village of Mainok in Borno state.
Troops managed to repel the attack with the help of reinforcements but it comes after a series of similar raids targeting soldiers that is likely to stoke fears of a resurgence.
Lawan Bukar Wasaram, a former chairman of the Kaga district where Mainok is located, said "large numbers" of Boko Haram militants tried to take over the base.
A leader of the civilian militia in the Borno state capital, Maiduguri, some 60 kilometres east of Mainok, gave a similar account, although he put the death toll at nine.
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"They attempted to overrun the military base but troops fought back. They had reinforcements," he added.
Motorist Laminu Isa was travelling to Maiduguri from the capital of neighbouring Yobe state, Damaturu, when he and hundreds of other drivers were caught up in the shooting.
"They came in eight pick-up trucks. We saw them when they crossed the roads and many of us stopped in panic. But they told us they weren't after civilians.
"Their main target was the military so they touched no- one among the motorists."
Isa said the gunmen came from the direction of Buni Yadi in Yobe, which links up with the Sambisa Forest in Borno -- the long-time base of factional leader Abubakar Shekau.
An increase in attacks, including against "hard" military targets, was expected as it has happened before when the Nigerian Army changed senior commanders in the northeast.
The army maintained it was a routine transfer but it followed increased violence in recent weeks, including a suicide bombings that caused heavy civilian casualties.
At least six soldiers died in two separate Boko Haram ambushes earlier this month near Damboa, south of Maiduguri.
The upsurge in attacks could also be an indication that Boko Haram is feeling pressure and is fighting back to secure weapons, ammunition, food and supplies.
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