Pirates have hijacked an oil products tanker off the coast of Nigeria after contact was lost with the vessel and crew over the weekend, the vessel's manager said on Thursday.

An official with Greek-based Ancora Investment Trust Inc said the Halifax tanker was still being held. They are holding the vessel and crew, the official told Reuters.

An oil security source told Reuters separately that pirates had hijacked a small oil supply boat, called MV Igbere, off the coast of Nigeria on Thursday.

The incidents were the latest in a lengthening string of attacks on ships in the Gulf of Guinea that experts say threatens an emerging trade hub -- and an increasingly important source of oil, metals and agricultural products like cocoa to world markets.

Pirates in the oil-rich Gulf of Guinea, which stretches from Guinea to Angola, tend to raid ships for cash and cargo rather than hijacking the crews for ransom like their counterparts off the coast of Somalia, analysts say.

What they are aiming at is the cargo, said the official from Ancora Investment Trust, which manages the tanker.

The official said the Halifax had been 60 miles off Nigeria's Port Harcourt before it was hijacked.

She was waiting to go inside and berth, the official said.

The Malta-flagged Halifax was last seen in the Bight of Benin, off the coast of Cotonou, with a destination showing Port Harcourt, AIS tracking data on Reuters showed.

If the vessel has been hijacked, it is likely to be held for a period of under 10 days until its attackers can offload as much of the cargo as possible before abandoning the ship, Security firm AKE Ltd said in a note on Thursday.

Vessel operators in the area are advised to proceed with caution, and ensure 24 hour watch rotas and best management practices are in place. Slow moving or stationary vessels are at an increased risk. The use of a correctly secured citadel remains an effective last measure of defence.

Earlier this month, pirates hijacked a chemical product tanker and kidnapped the crew off the Nigerian coast.