ABUJA (Reuters) - Armed pirates attacked an oil products tanker off the coast of Nigeria in West Africa and abducted an unknown number of crew, security sources said on Tuesday.

Increasing piracy in the Gulf of Guinea region, which includes Africa's No. 1 oil producer Nigeria and is a significant source of cocoa and metals for world markets, is jacking up costs for shipping firms operating there.

The Nigerian-flagged MT Matrix was boarded by gunmen in the early hours of Saturday about 40 nautical miles off the coast of oil-producing Bayelsa state, two security sources said, in a stretch of water often targeted by pirates.

There were 12 Pakistani and five Nigerian crew aboard the vessel when it was attacked, one of the sources said.

A spokesman for ship operator Val Oil Trading, who declined to give his name, confirmed there had been an "incident", without giving further details.

Andrew Varney, of British-based security firm Port 2 Port Maritime, said the Matrix's low freeboard - the distance between a ship's railings and the water - and slower speed made it vulnerable to being boarded.

"This latest incident further highlights the ability of these determined, adept criminals to attack vessels underway and the increasing migration from cargo theft to risk of kidnap for ransom," Varney said.

Last month there were two attacks in the Gulf of Guinea where foreigners were kidnapped and released a few weeks later. Security sources believe ransoms were paid - an increasingly lucrative business for criminal gangs.

"The risk of offshore kidnap for ransom remains high off Nigeria, particularly off Bayelsa and Rivers states," security firm AKE said, referring to the southern Niger Delta region where much of Nigeria's oil is produced.

International navies have not launched counter-piracy missions in the Gulf of Guinea, leaving the many vessels that anchor off Nigeria vulnerable to attack.

On the other, eastern side of Africa, piracy that was once a scourge off the coast of Somalia has been largely contained by international naval patrols and the stationing of armed guards aboard merchant vessels.

(Reporting by Joe Brock in Abuja and Jonathan Saul in London; Editing by Pravin Char)