Lt. Col. Sagir Musa said an "intelligence report" showed the head of the Islamist militant group may have died between July 25 and Aug. 3 after being shot June 30 during a military raid on a Boko Haram encampment in Sambisa Forest, the Daily Times of Nigeria reported Monday.
Musa said a video of Shekau released last Tuesday had been "dramatized by an impostor to hoodwink the sect members to continue with the terrorism."
The BBC said Boko Haram had not reacted to Musa's statement.
The United States had placed a $7 million reward on the head of Shekau, the leader of the group that has waged an insurgency in Nigeria since 2009,
The Nigerian military said it killed Boko Haram's No. 2 man, Momuda Bama, also known as "Abu Saad," last Wednesday.
No confirmation had been obtained of the deaths of either man.