YAOUNDE - Gunmen who kidnapped three Filipinos and a Ukrainian from an oil supply vessel contracted to Royal Dutch Shell <RDSa.L> off Cameroon three months ago are demanding $500,000 ransom, a local rebel leader told Reuters.

General A.G. Basuo, who describes himself as operations commander of the Bakassi Freedom Fighters rebel group, said the crew members were taken by militant fighters based in neighbouring Nigeria's lawless Niger Delta.

"Our men were not responsible for the attack, but ... we know the group who took the men hostage," Basuo told Reuters by telephone late on Wednesday. He said the hostages were in good health but declined to say where they were being held.

"They are now asking Pecten to pay them $500,000 before they can be released," he said.

Security contractors said on March 14 that some 30 suspected Nigerian militants in speedboats attacked the MV Sil Tide around 14 km from the coast of Bakassi, a peninsula jutting into the Gulf of Guinea on the Nigeria-Cameroon border.

Shell said the vessel was operated by oil services company Tidewater and was under contract to Pecten Cameroon, a joint venture in which Shell owns 80 percent and Cameroon's state oil company owns the rest.

There has been no claim of responsibility.

Attacks on oil vessels are common in Nigerian waters off the Niger Delta, home to Africa's biggest oil and gas industry. More than 200 foreigners have been taken in the delta over the past three years, most released unharmed after a ransom payment.

But gunmen in fast launches have increasingly struck beyond Nigerian waters, attacking oil installations, fishing boats, banks and even coastal towns around the Gulf of Guinea -- the main source of African crude exported to the West and China.

A senior Cameroonian military official, who asked not to be named, said there were fears that a military offensive by the Nigerians against militants in the Niger Delta could push rebel fighters into Cameroon.

"Our government is now worried that as the Nigerian government operation against the Niger Delta militants continues, they will take refuge in Bakassi, create a situation of chaos and instability and prevent efforts to intensify oil exploration activities," the official said.

Bakassi itself has also long been volatile.

The peninsula was handed over to Cameroon by Nigeria last year after a decades-long border dispute. Many Nigerian residents were opposed to the handover and militia groups, believed to be linked to Niger Delta gangs, sprung up.