Swarms can devour fields of crops in a few minutes

"There are signs that the situation is moving towards the early stages of an upsurge," said the FAO's Locust Group in a statement.

The group has called for international assistance to prevent a plague from developing in the region.

Locusts outbreaks have been reported in Mauritania, Niger and Sudan and now Morocco and Algeria are at risk.

An official at Morocco's national research and intervention centre has warned "as long as Mauritania does not have the locusts under control, we must remain vigilant".

"Our strategy is to block the locusts in desert areas so that they don't reach farming regions," Moha Bagari was quoted by AFP news agency as saying.

Borders

Desert locusts are normally solitary insects but when climatic conditions are favourable they can rapidly increase in number.

When they group, the young, wingless locusts - known as hoppers - march together in search of food.

As adult, winged insects they form swarms that can travel in tens of millions and travel great distances, crossing international borders in search of food.

They can devastate crops within minutes.

A major locust plague in 1987 to 1989 originated in western Sudan and spread as far as India.

The UN organisation says a full-fledged desert locust plague has the potential of damaging the livelihood of a tenth of the world's population.