Members of an alleged drug trafficking ring on trial in France are suspected of using used bullet-shaped "narco-torpedoes" bolted to the hulls of container ships to smuggle huge quantities of cocaine.

Thirteen people appeared in a court near Paris on Thursday (7 September) following a months-long investigation into the alleged drug trafficking operation.

The suspected drug gang is said by prosecutors to have been led by two men: a 39-year-old who smuggled cocaine from the Caribbean to France, and a 32-year-old who smuggled cannabis resin from Morocco.

The ring allegedly possessed fake passports, encrypted cell phones and cars registered in false names, AFP reported. They are also said to have rented apartments and parking spots in Paris.

Two of the members even spent €80,000 cash each to learn how to fly helicopters, prosecutors said.

Their alleged use of "narco-torpedoes" to smuggle illegal drugs across the Atlantic first came to the attention of the police in April 2014 when a plan to use divers on an underwater scooter to retrieve one of the "torpedoes" in the French port of Dunkirk was overheard by law enforcement agents.

After a subsequent attempt to bust the ring then failed, the investigation became active again when suspicious divers were spotted months later off the French Caribbean island of Guadeloupe.

A container ship headed for Spain was identified by police as being suspected of having a "narco-torpedo" attached to its hull.

It was instead found to be carrying a net holding 7kg of cocaine - just a fraction of an original shipment which is thought to have been lost at sea after the net tore open.

As the surveillance operation into the suspected gang's activities continued, some 120kg of cocaine was then found above deck on a container ship in Dunkirk in March 2015.

Lab tests determined the cocaine was similar to that found in the net.

The trial is expected to last a month. Six suspected members of the crime ring remain at large.