Two Nigerian Air Force personnel have been killed after their Mi-35 Hind multi-role combat helicopter crashed in the north-eastern state of Borno.
The helicopter was on a training mission and went down at a location south of Bama, after developing a technical fault.
One pilot was recovered alive, while another pilot and a technician were killed in the crash.
Nigerian Defence Headquarters information director major general Chris Olukolade said in a statement: "Investigation has commenced to unravel the circumstances that led to the accident.
"Further details on the crash will be made known as investigation progresses."
Bama is one of the areas in Borno State where the Boko Haram militant group has intensified its terrorist activities in the last few years.
There were suspicions that the helicopter might have been downed by the militants as they are believed to possess anti-aircraft weapons.
However, Olukolade ruled out the possibility of any attack.
"It is established that the crash is not as a result of any enemy action," Olukolade added.
"We are not conceding any portion of this country to any terrorist group. We are going to reverse every form of insecurity in that area very soon."
The crash comes even as Boko Haram insurgents attacked Damboa, another town in the troubled Borno state, killing several people and displacing more than 15,000 residents.
Damboa local authority spokesman Sheriff Muhammad told CNN: "We lost 46 people on Thursday through Friday in Damboa and the gunmen pursued people who fled to nearby Fori and Kwairi villages where they killed 20 more.
"Boko Haram have taken over Damboa where they erected their flags after the attack that forced all residents to flee."
The Nigerian military has also stepped up counter-terrorism and counter-insurgency offensive against Boko Haram to free more than 200 girls kidnapped from a secondary school in Chibok in April.