At least six people were killed in a bomb attack Friday at a mosque in Nigeria's northeastern town of Damboa, Borno state.

Nigerian Army spokesman Colonel Sani Usman said "two Boko Haram terrorists" were behind the attack.

Usman said the first attacker "targeted Damboa Central Mosque but due to stringent security measures he could not gain entry."

"Obviously frustrated, he exploded and died near the central mosque."

"However, the second bomber veered off and gained entry into another smaller mosque and detonated the bomb, killing himself and six other worshippers and injuring one other person."

A military source, who did not want to be named, earlier put the death toll at a different number, saying nine people were killed and over a dozen others injured in the attack.

There has been no claim of responsibility for the bombing.

Separately, the army spokesman said three other people were killed in an attack by suspected Boko Haram militants in the village of Gaskeri, near a camp for the internally displaced people, outside Borno's capital, Maiduguri, on Thursday night.

"They killed three civilian vigilantes and looted foodstuffs. Troops have been mobilized and they are on the suspected terrorists' trail."

Boko Haram started its campaign of militancy in 2009 with the aim of toppling the central government in Nigeria. It has so far taken the lives of at least 20,000 people and forced over 2.5 million others to flee their homes.

The terror group has pledged allegiance to the Daesh Takfiri terrorists.