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, the world's poorest country by the benchmark of the UN's Human Development Index (HDI), 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.

The defence ministry separately said Monday that "25 terrorists" were killed last week "from an air strike" in the western region of Tillaberi.