The Nigerian military has said that more than 260 Boko Haram militants have surrendered in north-eastern Nigeria.
A spokesman also said that the military had killed the man posing as the group’s leader, Abubakar Shekau.
Mohammed Bashir is said to have appeared in the group’s videos but is thought to be an imposter.
Boko Haram has suffered heavy losses in recent weeks as the Nigerian military battled the group close to its hometown of Maiduguri in the north east.
The military said that 135 Boko Haram members surrendered with their weapons in Biu, Borno State, on Tuesday – and that 133 others surrendered elsewhere in north-eastern Nigeria and were currently being interrogated.
The BBC’s Will Ross in Lagos says that this has never happened before in the war against Boko Haram.
Although it is impossible to independently verify, the military is seeing it as a turning point, our correspondent adds.
It was also the first time that the Nigerian military has publicly mentioned the death of the group’s leader Abubakar Shekau, who security forces claimed had died in clashes with the army in 2009.
General Chris Olukolade of the Nigerian military said that Mohammed Bashir, who was killed in the latest offensive against Boko Haram, was a lookalike.
Bashir “had been acting or posing in videos as the deceased Abubakar Shekau, the eccentric character known as leader of the group”, he added.
The military however did not give any dates or locations for when they believe leader Shekau actually died.
“Boko Haram” means “Western education is forbidden” in Arabic, and the group frequently attacks schools and colleges, which it sees as a symbol of Western culture.
Boko Haram was behind the kidnapping of more than 200 schoolgirls from Chibok in Borno state in April.
The New York-based Human Rights Watch says more than 2,000 civilians have been killed in the region this year.