Swarms of desert locusts, which have been ravaging crops and grazing land across east Africa, have now crossed the border into South Sudan, a country already struggling from widespread hunger and years of civil war.

The UN has warned that an imminent second hatch of the insects could threaten the food security of 25 million people across the region.

According to South Sudan’s ministry of agriculture, mature locusts looking for places to breed have been confirmed in three locations after the insects crossed into southern Magwi county, on the border with Uganda, and are now moving west.

Confirmation of the presence of the locusts, part of a once in a generation event that has produced swarms the size of cities, came after a joint team from the South Sudanese ministry of health and the UN’s Food and Agriculture Organization visited the area.

“Yesterday we received the report that desert locusts have entered South Sudan from Magwi county and our team and those of [the] FAO sent people there, and they have confirmed the presence of locusts in the area,” said the country’s agriculture minister, Anyoti Adigo Nyikwach.

The appearance of the locusts follows a period of extreme weather, including devastating floods, that have further threatened the food supply. Experts are warning that the main March-to-May cropping season is now at risk. The situation, however, remains most serious in Kenya, Ethiopia and Somalia amid evidence that aerial spraying of pesticides against the swarms has so far had very limited impact.

Desert locusts are seen mating in Lekiji, in Kenya’s Samburu County. Photograph: Sven Torfinn/FAO

Desert locusts can travel up to 95 miles in a day and can eat their own body weight in plant material, meaning even a small swarm can consume as much food as 35,000 people in a day, according to the UN.

Kenya, Somalia, Eritrea and Djibouti were already battling the worst locust outbreak in decades, with swarms also spreading into Tanzania and Uganda.

Meshack Malo, South Sudan’s representative for the FAO, said the locusts were mature and looking for breeding grounds that would form the basis of the next major infestation, which it is feared will occur later this month.

“These [locusts] are deep yellow, which means that they will be here mostly looking at areas in which they will lay eggs,” he said.

According to the FAO’s locust watch monitor, widespread breeding is now underway across the Horn of Africa.

Teams planned to mark the place where they lay eggs and then come back to kill the young insects in 14 days, Malo said, since poisoning the eggs in the ground could damage the soil.

At least 2,000 locusts have so far crossed the border, he said. During each three-month breeding cycle, a single locust can breed 20 more, giving rise to the massive swarms that are now threatening crops on either side of the Red Sea.

Oil-rich South Sudan is recovering from five years of civil war that plunged parts of the country into famine in 2017 and forced a quarter of the population to flee their homes.

In December, the UN’s World Food Programme said the food security outlook was dire after floods affected nearly 1 million people, with some 60% of the population facing food insecurity.