At least 26 members of the Nigerian security forces were killed and eight wounded in an ambush by gunmen in central Nigeria, two military sources have said.
An air force spokesperson said a helicopter rescuing the wounded crashed on Monday morning in the area where the army is fighting criminal groups, without specifying whether the crew and passengers had survived.
In a statement, Edward Gabkwet, the spokesperson for the air force, said the MI-171 helicopter crashed near Chukuba village in the Shiroro local government area.
Barely a week goes by in Africa's most populous nation without attacks or kidnappings by criminals known as "bandits" in the northwest and centre.
Impunity as well as insufficient security and wider government presence has allowed the violence to fester, experts say.
Western officials and observers have commented that last month's coup in Niger came at a precarious time.
The West African regional bloc ECOWAS has approved the deployment of a "standby force to restore constitutional order" in Niger.
But ECOWAS has yet to provide details on the force or a timetable for action, and the leaders have emphasised they still want a peaceful solution.