A swarm of locusts measured at 37 miles long and 25 miles wide has been tracked in Kenya - and the insects are now threatening to decimate crops across swatches of east Africa.

The most serious outbreak of desert locusts in 25 years is 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.

Kenya's Intergovernmental Authority on Development said: “A typical desert locust swarm can contain up to 150 million locusts per square kilometre.

“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.”

A Samburu boy uses a wooden stick to try to swat a swarm of desert locusts filling the air, as he herds his camel near the village of Sissia, Kenya. Credit: AP

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

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

The "extremely dangerous" outbreak is making the region's bad food security situation worse, the United Nations’ Food and Agriculture Organisation has warned.

Hundreds of thousands of acres of crops have been destroyed.

A man holds a desert locust, considered to be the most dangerous of the locust species. Credit: AP

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

Major locust outbreaks can be devastating.

A major one between 2003 and 2005 cost more than $500 million to control across 20 countries in northern Africa, the FAO has said, with more than $2.5billion in harvest losses.

To help prevent and control outbreaks, authorities analyse satellite images, stockpile pesticides and conduct aerial spraying. In Ethiopia, officials said they have deployed four small planes to help fight the invasion.