Swarms of desert locusts that have been devouring crops and pasture in the East Africa region have spread to South Sudan, the UN food agency says.
The pests entered via Uganda into the southern county of Magwi.
Several million people in South Sudan are already facing hunger as the country struggles to emerge from a brutal civil war.
The UN has warned that a food crisis could be looming if the outbreak is not brought under control.
The invasion is the worst infestation in Kenya for 70 years and the worst in Somalia and Ethiopia for 25 years.
Efforts to control the locust infestation have so far not been effective.
Aerial spraying of pesticides is the most effective way of fighting the swarms but countries in the region do not have the right resources.
There are now fears that the locusts – already in the hundreds of billions – will multiply further.
The Food and Agriculture Organization (FAO) says the insects, which eat their own body weight in food every day, are breeding so fast that numbers could grow 500 times by June.
The UN body last week called on the international community to provide nearly $76m (£58m) to fund the spraying of the affected areas with insecticide.
An average swarm can destroy crops sufficient to feed 2,500 people for a year, according to the FAO.
Somalia has declared a national emergency in response to the crisis.
The Ethiopian government has called for “immediate action” to deal with the problem affecting four of the country’s nine states.
Kenya has deployed aircrafts to spray pesticides in several regions and its Agriculture Minister Peter Munya said on Monday that the invasion was “under control”.
Meanwhile, Uganda has deployed soldiers to the northern regions to spray pesticides in the affected areas.
Egypt and Sudan have also been affected.
The locust swarms entered Africa from Yemen three months ago.