More than 260 Boko Haram militants have surrendered in north-eastern Nigeria, the military has said.

The military says it has also killed a man featuring in the group's recent propaganda videos, pretending to be its deceased leader Abubakar Shekau.

The army said it had killed Shekau last year, but never provided proof.

Boko Haram has suffered heavy losses in recent weeks as the Nigerian military battles 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.

The BBC's Will Ross in Lagos says that the claims are impossible to verify.

However, if they are true, it could be a major turning point for the army's campaign against Boko Haram militants, 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 a man named Mohammed Bashir was among those killed in the latest offensive against Boko Haram last week.

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 Shekau actually died.

Speaking at the United Nations Security Council meeting, Nigerian President Goodluck Jonathan urged the council to find more ways to combat the militant threat.

"Evidence has shown that Boko Haram is sourced largely from outside our country," he said.

"Only by united action and firm resolve can we check this urgent threat to humanity and also build the enduring structures that will resist their re-emergence."

"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.