A Nigerian warplane crashed in Niger on Monday, from where it was conducting military operations in neighbouring Mali, and the two pilots were killed, Nigeria's army said in a statement.

The fighter jet was part of Nigeria's air fleet engaged with an African force battling hardline Islamists in Mali. An army source in Niamey said it had suffered a "mechanical" problem.

"A Nigerian Air Force Alpha jet aircraft on a non-combattant mission crashed today at Dargol village...The two pilots on board the fighter jet died," the Nigerian defence headquarters said in a statement.

The jet is one of four Nigerian aircraft based in Niamey as part of of the Africa-led International Support Mission to Mali (AFISMA), it said, adding that the crash was being investigated.

French and Chadian planes and US drones have used the Niamey airport as a base for their operations in Mali.

The 6,000-strong AFISMA has taken over the lead from France in supporting Mali's army in the battle against Al-Qaeda-linked extremists.

A French-led operation launched in January drove the Islamists from key cities they had seized in northern Mali. But the Islamists have turned to suicide bombings and guerrilla attacks in reclaimed territory.

AFISMA will eventually be incorporated into a 12,600-strong peacekeeping force, following a UN resolution in April.