A devastating plague of caterpillars ravaging part of West Africa is not armyworms, as previously believed, but an unidentified species, experts say.

A UN emergency co-ordinator told the BBC the insects in Liberia and Guinea were very different from armyworms.

He said experts had noted the insect has distinct feeding patterns, life cycle, habits, movement and appearance.

Specialists are studying the pest to find a way of controlling the swarm, which has affected 400,000 residents.

As well as devouring crops, the infestation has polluted water sources with faeces.

UN Food and Agriculture Organisation emergency co-ordinator Tim Vaessen said insect experts had realised the species was not armyworm during a field trip to Bong County in Liberia last week.

While armyworms feed on ground cereal like millet, rice or sorghum, he said, the unidentified insect favours munching the leaf of the Dohama tree.

Mr Vaessen said the unknown species' cocoons are found on the surface beneath ground leaves, whereas armyworm casings are buried in the soil at a depth of several centimetres.

He also said the mystery caterpillar appears to rear up, making half circles in the air as it moves forward, which the armyworm does not do. The wings of its moth also appear different.

Mr Vaessen told the BBC News website: "Our experts in the field spoke to villagers who said they'd seen this type of caterpillar before. They said they'd put leaves under trees and burned them to suffocate the caterpillar with smoke.

"But the villagers said they've never seen it in such large numbers before, they're really flabbergasted."

The FAO has sent pictures of the insect to experts at the International Institute of Tropical Agriculture in Benin and at Cardiff University as part of efforts to help identify the species.