The most serious outbreak of desert locusts in 25 years is spreading across East Africa and posing an unprecedented threat to food security in some of the world’s most vulnerable countries, authorities say.

Unusual climate conditions are partly to blame.

The locust swarms hang like shimmering dark clouds on the horizon in some places.

Roughly the length of a finger, the insects fly together by the millions and are devouring crops and forcing people in some areas to wade through them.

An “extremely dangerous increase” in locust swarm activity has been reported by officials in Kenya this week.

One swarm measured 37 miles long by 25 miles wide in the north-east of the country, the Intergovernmental Authority on Development (IGAD) said in a statement.

“A typical desert locust swarm can contain up to 150 million locusts per square kilometre,” it said. “Swarms migrate with the wind and can cover 100 to 150 kilometres in a day. An average swarm can destroy as much food crops in a day as is sufficient to feed 2,500 people.”

The outbreak of desert locusts, considered the most dangerous locust species, also has affected parts of Somalia, Ethiopia, Sudan, Djibouti and Eritrea and the IGAD warned that parts of South Sudan and Uganda could be next.

Locust outbreak
Locusts are roughly the length of a finger (Patrick Ngugi/AP)

The outbreak is making the region’s bad food security situation worse, the UN Food and Agriculture Organisation has warned. Hundreds of thousands of acres of crops have been destroyed.

Already millions of people cope with the constant risk of drought or flooding, as well as deadly unrest in Ethiopia, extremist attacks in Somalia and lingering fighting in South Sudan as it emerges from civil war.

The further increase in locust swarms could last until June as favourable breeding conditions continue, the IGAD said, helped along by unusually heavy flooding in parts of the region in recent weeks.