The accident happened when the Russian-built Mi-17 crashed as it was landing at a military base at Niamey airport.
"Unfortunately, all three crewmembers -- a Nigerien officer and NCO and an expatriate instructor -- died immediately despite efforts by rescue services to put out the fire," it said in a statement.
An investigation has been launched, the statement said.
Niger has vowed to boost its military in the face of jihadist insurgencies in the southwest and southeast of the country.
The country is receiving training and logistical support from France and the United States, which have major air bases in Niger, and from Belgium, Italy and Germany.
Separately, the defense ministry said Monday that "25 terrorists" were killed last week "from an air strike" in the western region of Tillaberi.
Niger 'Copter Crash Kills Three
FILE: A soldier of the French Special and Intervention Unit of the National Gendarmerie stands next to a NH90 Caiman Helicopter at the Ouallam's military base on July 15, 2022
Three people died on Monday when a Nigerien military helicopter crashed on return from a routine training flight, the defense ministry said.
NIAMEY —