Local swarms were first reported in January near the Libyan town of Ghat.
Ongoing rains and the resulting growth of vegetation along the southern fringes of the Sahara led to an increase in locust populations by mid-May, the agency reports.
The FAO also cautions that the recent coup and rebel insurgency in Mali could hamper efforts to control the ravenous insects.
The region last faced desert locust swarms during a 2003-2005 plague that affected farmers in two dozen countries across the Sahel.
"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 Keith Cressman, FAO Senior Locust Forecasting Officer.
Photo: U.N. Food and Agriculture Organization