Pirates have boarded a ship off the coast of Nigeria and kidnapped six Russians and an Estonian, in the latest such incident in the region, the French company Bourbon has said.

"Bourbon confirms that seven crew members ... were kidnapped during the boarding of the Bourbon Liberty 249, which occurred on October 15, 2012 in Nigeria," a statement from the oil industry servicing firm said.

"The other nine crew members are still on-board the vessel which is heading for the Port of Onne. They are safe and sound, and in good health."

The statement did not give the location of the abduction, but Onne is located in the Niger Delta, the country's oil-producing region.

Pirate attacks are on the rise in the Gulf of Guinea, which is second only to the waters around Somalia in terms of piracy.

Usually boats are attacked in order to steal their cargo, after which the crew is normally freed, although kidnapping is also a major criminal enterprise in the waters around the delta - the heart of Africa's biggest energy industry.

Pirates freed a Greek-operated gasoline tanker earlier this month that they had hijacked in the Gulf of Guinea near the Ivory Coast. Fuel ships are a favourite target.

Many of the criminal gangs in pirate networks are offshoots from armed groups that used to operate in the delta before they agreed to an amnesty deal in 2009 - which greatly reduced unrest there.

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 after stealing 3,000 tonnes of fuel.