Another nine crew members were safe and had reached the Nigerian port of Onne, in the oil-rich Niger Delta, said the spokeswoman for Bourbon, which supplies vessels to the offshore oil industry. They were abducted on Monday.

Pirate attacks are on the rise in the Gulf of Guinea, which is second only to the waters around Somalia for piracy. Usually boats are attacked in order to steal their cargo, after which the crew is normally freed.

Kidnapping is also a major criminal enterprise in the waters around the delta.

Pirates freed a Greek-run gasoline tanker this month that they had hijacked in the Gulf of Guinea near Ivory Coast. Fuel ships are often targets.

Many of the criminal gangs in pirate networks are offshoots from militant groups that used to operate in the delta before they agreed an amnesty in 2009.

In August pirates attacked a Greek-operated oil tanker with a crew of about 20 off the coast of Togo. They released the ship a few days later.