A plague of caterpillars munching their way through crops in northern Liberia could spread across West Africa, causing a food, health and environmental emergency, the U.N. Food and Agriculture Organisation (FAO) warned on Thursday.

The agency said the ravenous insects, between 2 and 3 cm in length, were advancing in the "tens of millions", devouring all plants and food crops in their path.

Described by local people as "black, creeping and hairy", they have also overrun buildings, forcing villagers to flee their homes.

Some communities cannot reach their farms because they are surrounded by the pests, which pose a major threat to food security, the FAO said.

FAO Liberia representative Winfred Hammond said the situation was a national emergency. He warned that, unless the invasion was contained quickly, it would "very likely" escalate into a regional crisis involving neighbouring Guinea, Sierra Leone and Ivory Coast.

There are reports the caterpillars, thought to be African army worms, are heading across the border into Guinea.

So far they have attacked 46 villages in Liberia's Bong, Lofa and Gbarpolu counties, in what is being described as the worst infestation in 30 years. Bong county is the hardest-hit area, with some two-thirds of its 200,000 inhabitants affected.

The insects' droppings are also contaminating wells and waterways, making the water undrinkable.

Hammond said Liberia would need international help to deal with the crisis because it does not have enough financial resources and technical expertise to fight the problem on its own.

Three emergency committees have been set up to plan the response, mobilise resources and provide information. An FAO task force will also assess the situation and draw up urgent and longer-term measures.

The infestation is spreading fast because the caterpillars are breeding rapidly and adult moths can fly long distances at night, the FAO said.

Pesticides sprayed by hand have failed to stamp them out because many live in giant forest trees more than 8 metres above ground.

But the FAO said it did not recommend aerial spraying of pesticides because that method could further contaminate precarious water supplies.

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