Heavily-armed pirates swarmed aboard an oil industry support vessel working off the coast of Cameroon and kidnapped 10 crew members, including six Frenchmen, the French foreign ministry said Friday.
Following the pre-dawn attack, a man claiming to represent a rebel group opposed to Cameroon's takeover of the Bakassi Peninsula warned the hostages would be killed unless Cameroonian officials agreed to reopen the issue.
"The 10 are in our hands. If you don't tell the government of Cameroon to come here and discuss with us, we will kill them all in three days," the self-styled "Bakassi Freedom Fighters" said told AFP.
Nigeria ceded Bakassi to Cameroon in August after a ruling by the International Court of Justice brought to an end a 15-year dispute over the peninsula, including rights to its oil fields and fishing grounds.
The handover was completed peacefully, but some local groups opposed the change of sovereignty and have threatened attacks.
The Bakassi Freedom Fighters, part of a shadowy Nigerian group dubbed the Niger Delta Defence and Security Council, claimed responsibility in June and July for attacks that killed seven Cameroonian troops and a local official.
The Bourbon Sagitta was stormed as it took part in an operation to load crude oil onto a tanker in the Gulf of Guinea, near Cameroon's maritime frontier with Nigeria, the shipping firm Bourbon told AFP in Marseille.
"Armed individuals aboard two 'flying boats' boarded the ship and took 10 of the 15 members of the crew," a spokesman said, using the term employed in west Africa for the fast fibreglass skiffs typically used by pirate gangs.
Along with the Frenchmen, a Tunisian, a Senegalese and two Cameroonians were also taken, the company spokesman said. No-one was thought to have been hurt in the raid, and the five remaining sailors have been safely recovered.
"The crisis centre and the French consulate in Douala are in contact with the company employing our citizens and with their families," the French foreign ministry in Paris said in a statement.
A diplomat in the Cameroonian capital Yaounde said the attack bore the hallmarks of raids carried out by Nigerian pirates operating in the Niger Delta oilfields.
The Bourbon Sagitta is a 2,000-tonne, 65-metre (213-foot) Anchor Handling Tug Supply Vessel, a type of ship used to haul anchors and drilling equipment in offshore oil fields. It was built in China's Dayang shipyard in 2006.
The Bakassi Peninsula is a 1,000-square-kilometre (386-square-mile) strip of coastal swamp jutting out from the Cameroon-Nigeria border into the oil- and fish-rich waters of the Gulf of Guinea.
Ownership of the area has not been clear since the colonial era, and in 1993 Nigerian troops occupied much of the zone and set up a local administration.
Cameroon protested and took its claim to the International Court of Justice in the Hague in March 1994, starting a long legal battle that ended in October 2002 when the court awarded it sovereignty.
Nigeria did not dispute the judgment and, after a period of border demarcation punctuated by occasional deadly skirmishes, ceded control in August this year, to the dismay of many local communities.