A shipping security expert says the crew of an oil tanker hijacked in Nigerian waters is from Russia, Georgia and the Philippines and that they are probably being held hostage for ransom.

Dirk Steffen of Denmark-based Risk Intelligence says claims that the Greek-owned MT Leon Dias was boarded by separatists espousing an independent state of Biafra in southeast Nigeria likely is a cover to lend legitimacy to a kidnapping for ransom.

He says the hijackers abandoned the ship Sunday and took five hostages with them. They include the Filipino captain and third engineer, the chief engineer and electrician from Russia and a fitter from Georgia.

Nigeria's navy said the ship is now off the coast of neighboring Benin, guarded by that country's navy.