At least 36 Nigerian soldiers were killed in two attacks during operations against armed gangs in the northern-central state of Niger, the military said on Thursday.
Gangs of heavily armed men have wreaked havoc across northwest Nigeria in the past two years, kidnapping thousands, killing hundreds and making it unsafe to travel by road or to farm in some areas.
Defence spokesman Major-General Edward Buba said the fatalities include three officers and 22 soldiers, with seven soldiers wounded during an ambush around Kundu village in the Shiroro local government area in Niger state on August 14.
An Air Force Mi-171 helicopter dispatched to evacuate the casualties crashed near Chukuba village in Shiroro on Monday, killing others.
While Buba didn't confirm the cause of the crash, two military sources told Reuters the helicopter was probably brought down after gang members shot at it.
"The helicopter crashed with 14 of the earlier killed in action personnel, seven of the earlier wounded in action personnel, two pilots and two crew members," Buba said in a statement.
"Operations are ongoing to recover the bodies and investigate the cause of the air crash which will be communicated," he said.
Attacks by gangs, often referred to locally as bandits, have confounded Nigeria's security forces, who have been stretched as they tackle a violent secessionist group in the east, a deadly herder-farmer crisis in the central states, and a 13-year insurgency against militant groups Boko Haram and Islamic State West Africa Province (ISWAP) in the north.
In a separate military operation in Kaduna state, troops rescued 10 hostages held in a village in Igabi local government of the state following a tipoff, killing their abductors after a gun battle, Buba said.