Pirates have hijacked an oil tanker off the coast of Nigeria, in the Gulf of Guinea.

Initial reports on the hijacking are sketchy, but agree the tanker, the MT Halifax, was seized in waters near Port Harcourt, Nigeria.

It was not immediately clear how many crew members are on board. The Malta-flagged tanker is managed by Ancora Investment Trust of Greece.

Pirate hijackings have increased this year in the Gulf of Guinea, located off the southern coast of West Africa.

Analysts say pirates in the area tend to raid ships for cash and cargo rather than holding crew members for ransom.

Security officials say it is likely the pirates who hijacked the Halifax will try to offload the oil cargo while the tanker is out at sea.