DAKAR (Reuters) - A French navy warship has intercepted a Liberian-flagged fishing vessel carrying 2.5 tonnes of cocaine in waters off the West African coast, a United Nations anti-narcotics official said on Thursday.

It was the biggest cocaine seizure so far this year off West Africa, which has increasingly become a trans-shipment point favoured by Latin American drug cartels because of weak local law enforcement and its long largely unsupervised coastline.

In this growing trafficking corridor, the drugs are flown or shipped across the Atlantic and then on to markets in Europe.

The Blue Atlantic and its nine Ghanaian crew members were detained by the French navy vessel Tonnerre on Wednesday, 520 km south west of the Liberian capital Monrovia, Amado de Andres, deputy representative for West Africa of the U.N. Office on Drugs and Crime (UNODC), told Reuters.

"When the crew saw they were going to be intercepted, they started throwing containers of the drug overboard," he said.

Nevertheless, 2.5 tonnes of cocaine in more than 90 containers were seized.

De Andres said the French navy made the interception thanks to information supplied by the newly-created European Union anti-narcotics unit known as MAOC-N (Maritime Analysis and Operations Centre-Narcotics), which is based in Lisbon.

The U.N. and western law enforcement agencies have raised the alarm about the drug cartels' use of West Africa as a staging post, especially in the small state of Guinea-Bissau whose police are badly equipped and poorly paid.

"The traffickers are now trying to target new states and one of them is Liberia," de Andres said.

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