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The UN has said that if an Ethiopian locust infestation is not brought under control then the crop-devouring insect could "invade" neighbouring countries.

The Ethiopian government has called for "immediate action" to deal with the problem affecting four of the country's nine states, a UN statement said.

In northern Amhara state, it added.

Efforts to control the infestation have so far not been effective.

The authorities' work has also been affected by "ongoing insecurity" in some areas of the country, according to the UN's Food and Agricultural Organization (FAO).

"We need to act fast and mobilise the required resources urgently to scale up control and preventive measures," FAO Ethiopia representative Fatouma Seid said.

Last week, the government said it had sent planes to the affected areas to try to deal with the problem from the air.

Overall, the desert locusts have devoured crops and pasture across a swathe of land covering parts of the Tigray, Amhara, Oromia and Somali regional states.

The FAO estimated that the insects were eating 1.8 million tonnes of vegetation a day across 350 sq km (135 sq miles) of the country.

If additional control measures fail then the locusts "could continue moving within Ethiopia and invade" north-eastern Kenya, parts of Eritrea, and Sudan's southern coast, the FAO said.

The locusts are thought to have spread from Yemen three months ago.