Jacques Klein said UN planes had spotted about a dozen vessels, all of which had hidden their name-plates. None was flying a flag and they were dragging 10 to 15 kilometre nets.

"They were scooping up everything there was to scoop up," Klein told Liberia's UN radio station.

"The ships scattered when they saw the UN aircraft. A follow up later found similar illegal activity which, because it involves toxic substances, hurts not only the environment but also the economy of one of the worlds poorest countries," he said.

Klein said he did not have the authority to force the ships in to harbour.