The FAO has so far raised $22m (£16.8m) out of the $76m it has appealed for to launch a large scale response to the locust infestation.
Desert locust swarms are now multiplying across Somalia, Kenya and Ethiopia and more have already been observed in Eritrea, Djibouti and northeastern Uganda, the regional Food Security and Nutrition Working Group warned on Friday.
"Looking forward, given favourable forecast weather conditions, swarms are expected to increase in areas already affected, as well as spread to neighbouring areas," the update said.
"There is also a high risk that locusts will spread to South Sudan."
Several million people in South Sudan are already facing hunger as the country struggles to emerge from a civil war.
The FAO also expects locusts to breed and spread in coming months as the March-April start of the long rains is forecast to encourage the locusts to breed again and spread even further.
This is the second warning this week, after the UN's humanitarian chief Mark Lowcock said a region where 13 million people already face severe food insecurity cannot afford another jolt.
The UN warned: "We simply cannot afford another major shock."