Longtime Boko Haram leader Abubakar Shekau appeared in a video denying military reports that he was killed last month, and mocking parents of the hundreds of Chibok schoolgirls kidnapped in 2014.
In the video, Shekau claimed the parents would see their daughters again only if the government swapped them for detained terror leaders. "To the people of Chibok: You have not seen the worst yet," Shekau said, ending with gales of laughter.
The Islamic State in August appointed a new leader in Nigeria, but Shekau has insisted he remains in charge.
Nigeria's military called the video "the handiwork of a mentally sick and unstable mind." The military several times in the past has claimed to have killed Shekau, only to have him reappear.
Meantime, Islamic extremists killed eight soldiers and wounded 17 in two attacks in northeastern Nigeria, the country's military said Monday.
The attacks brought the official toll to 10 troops killed and 24 wounded this past week after a lull during which the extremists confronted a leadership struggle. The Islamic State group's West Africa Province claimed more than 40 soldiers from a multinational army were killed in one attack last week.
Extremists attacked an army position Sunday at Logomani, 70 miles northeast of Maiduguri, northeastern Nigeria's biggest city, an army statement said. The troops killed 22 fighters while losing four soldiers, it said.
Later Sunday, extremists ambushed a military convoy near Bama, southeast of Maiduguri, the statement said. An officer, three soldiers and three insurgents died.
Bama is a remote desert post where Borno state Gov. Kashim Shettima has set up temporary headquarters to encourage displaced people to return home and to speed up reconstruction of the town destroyed when it was occupied for months by extremists.
The Associated Press contributed to this report.