Political insecurity and conflicts in North Africa are hindering efforts to control swarms of desert locusts that threaten crops in Niger and Mali, the UN's food agency said on Tuesday.

"Croplands in Niger and Mali are at imminent risk from desert locust swarms that are moving southward from Algeria and Libya," the Food and Agriculture Organisation (FAO) said, warning locusts had already been found in north Niger.

"Locust control efforts in the region are being hindered by continued insecurity along both sides of the Algerian-Libyan border. Political insecurity and conflict in Mali could also hamper monitoring and control," it said.

Locust infestations were first reported in southwest Libya near Ghat in January 2012 and in southeast Algeria.

While the Rome-based agency had warned in March that swarms could arrive in Niger and Mali by June, it said Tuesday that persistent rains and the resulting boom in vegetation had accelerated the process.

"How many locusts there are and how far they move will depend on two major factors -- the effectiveness of current control efforts in Algeria and Libya and upcoming rainfall in the Sahel of West Africa," said FAO officer Keith Cressman.

"In a normal year, Algeria and Libya would have been able to control most of the local swarms and prevent their movement towards the south," he said.

"But insecurity along both sides of the Algerian-Libyan border is getting in the way of full access by local teams and by FAO experts," he said, adding that Libya's capacity to carry out controls had also been affected.

Niger last faced desert locust swarms during the 2003-05 plague that affected farmers in two dozen countries.