Boko Haram fighters killed at least 50 Nigerian soldiers in an ambush near Goneri village in northern Yobe state, according to military officials and villagers.
"The Nigerian military suffered some casualties in the unfortunate attack," defence ministry spokesman John Enenche told journalists in the capital, Abuja, on Tuesday.
Enenche did not disclose the exact number of casualties, but local witnesses who spoke to German press agency dpa on the telephone placed the death toll at between 50 and 75 people.
The ambush occurred near the town of Goneri in eastern Borno State on Monday, according to Enenche.
The attack, one of the deadliest against troops recently, came as the military tried to launch an offensive against the armed group that began over the weekend, a military official with knowledge of the operation told The Associated Press. The official spoke on condition of anonymity because he was not permitted to speak to the media about the matter.