Monrovia - Mystery on Tuesday surrounded the swarm of caterpillars in Liberia which have devastated crops and prompted an official state of emergency, after officials admitted they could not identify them.

"They are not army worms, though we haven't determined what they are," Agriculture Minister Chris Toe told a news conference in the West African state's capital on Monday.

The head of the team set up by the government to investigate and contain spread of the pests, Dr J Qwelibo Subah, confirmed they are yet to identify the species.

The creatures emerged almost a month ago in the remote Bong, Lofa and Gbarpolu counties and are now threatening the food security of around 350,000 people.

Agriculture experts warn the worms, which are spreading rapidly and have already been sighted in Guinea, could spread into Ivory Coast and Ghana.

Subah said samples of the creature have been dispatched to countries including Ghana and Britain for testing.

The United Nations Food and Agriculture Organization (FAO) last week said 1.2 million dollars was needed to help combat the spreading swarm.

It warned that if nothing was done further waves of worms could have an even more devastating impact on food crop production.

"We need to be prepared for a secondary and tertiary wave of the pests," Toe told journalists. (dpa)