Gunmen attacked an oil supply vessel contracted to Royal Dutch Shell off Cameroon near the border with Nigeria on Saturday, kidnapping one Ukrainian and three Filipino crew, security sources said.

Two security contractors said some 30 suspected Nigerian militants in speedboats attacked the MV Sil Tide on Saturday morning around 14 kilometres from the coast of Bakassi, on the Nigeria-Cameroon border.

"Around five shots were fired. One person was injured and they got the three Filipinos and the Ukrainian," one of the security sources, who asked not to be named, told Reuters.

Both sources said the attack happened in Cameroonian waters. One said the vessel was under Cameroonian naval escort.

A spokesman for Shell said the vessel was operated by oil services company Tidewater and was under contract to Pecten Cameroon, a joint venture betweenShell and Cameroon's state oil company.

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

Cameroon, Nigeria, Equatorial Guinea and Benin have all seen such attacks in recent years.

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

Gunmen in motor boats attacked the presidential palace in Equatorial Guinea's island capital Malabo last month. The government said it believed the attackers came from Nigeria.

The Bakassi peninsula near where the four foreigners were taken on Saturday has long been a centre of unrest.

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

Two allied militia groups from Bakassi seized 10 mostly French crew members from a vessel operated by oil services firm Bourbon off Cameroon last October. The hostages were released unharmed around 10 days later.

Report: Reuters - Additional reporting by Tansa Musa in Yaounde and Austin Ekeinde in Port Harcourt; Editing by Katie Nguyen