A man purporting to be the leader of Nigerian Islamist militant group Boko Haram, Abubakar Shekau, denied the jihadist group has been pushed out of its stronghold in the Sambisa forest, but the army said the base had been captured.

President Muhammadu Buhari on Saturday said Boko Haram's last enclave in the forest, a former game reserve in northeastern Nigeria, had been captured in the "final crushing" of the group.

Reuters has been unable to independently verify that the area was captured.

"We are safe. We have not been flushed out of anywhere,"

the man identifying himself as Shekau said in a video seen by Reuters on Thursday.

"If you indeed crushed us, how can you see me like this? How many times have you killed us in your bogus death?" he added in the 25-minute video during which he spoke in the Hausa language and Arabic.

Nigeria's military has in recent years said it has killed or wounded Shekau on multiple occasions.

Such statements have often swiftly been followed by video denials by someone who says he is Shekau, but poor footage makes it hard to confirm if the person is the same man as in previous videos.

Army spokesman Sani Usman said "spurious claims" were made in the latest video.

"The Nigerian army wishes to reiterate that it has captured and occupied the last known stronghold of the terrorist group in the Sambisa forest," he said.

"This video is nothing but mere terrorist propaganda aimed at creating fear in the mind of people and to remain relevant," Usman said.

Boko Haram has killed about 15,000 people and displaced more than 2 million in a seven-year insurgency aimed at creating a state adhering to strict Islamic laws in the northeast of Africa's most populous nation.

In early 2015, it controlled an area about the size of Belgium. But it has been pushed out of most of that territory over the past year by Nigeria's army and troops from neighbouring countries, moving to a base in the Sambisa.

Security analysts say the group's ability to carry out attacks in neighbouring Niger, Cameroon and Chad suggests it has multiple bases.

They also say the group split this year, with one faction led by Shekau operating from the Sambisa and the other, allied to Islamic State and led by Abu Musab al-Barnawi, based in the Lake Chad region.

Reuters