* Ship disappeared off southwest African state Jan. 18
* Greek owners say pirates stole large quantity of cargo
* Angolan navy denies piracy, says crew faked attack
* Incident raises fear piracy spreading south from W.Africa (recasts with owners' statement on pirate attack)
LISBON/ATHENS Jan 26 (Reuters) - The Greek owners of an oil tanker that vanished off the Angolan coast on Jan. 18 said on Sunday that pirates had hijacked the vessel and stolen a large quantity of cargo, contradicting the Angolan navy's denial that such an assault took place.
Greece-based Dynacom, owners of the 75,000 deadweight tonne Liberian-flagged tanker MT Kerala, said it had managed to contact crew on the vessel who reported the pirates had left.
"Pirates hijacked the vessel offshore Angola and stole a large quantity of cargo by ship-to-ship transfer. The pirates have now disembarked," the company said in a statement.
It did not provide any further details on the attack or the ship's current location but added that all crew were safe.
Dynacom's version of the events contradicted an account from the Angolan navy, which alleged the crew had turned off the ship's communications to fake a pirate attack.
Captain Augusto Alfredo, spokesman for the Angolan navy, told Reuters earlier on Sunday that the ship had been located in Nigeria and that reports of a hijacking were false.
The reports raised concern that piracy off West Africa was spreading south from the Gulf of Guinea, near Africa's biggest oil producer Nigeria, where most hijacking gangs are believed to originate.
Pirate attacks jumped by a third last year off West Africa. Any attack off Angola, which is the continent's No. 2 crude producer, would be the most southerly to date.
"It was all faked, there have been no acts of piracy in Angolan waters," Alfredo told Reuters. "What happened on Jan. 18, when we lost contact with the ship, was that the crew disabled the communications on purpose."
Alfredo declined to comment on how the navy had established the behaviour of the MT Kerala's crew, saying only that other authorities may provide further details later.
He also would not be drawn on the crew's possible motivation but said the ship was due to finish a time-charter contract for the Angolan state oil firm Sonangol on Feb. 12.
Sonangol said on Friday the MT Kerala had 27 crew, all of them Indian or Filipino.
Alfredo said a tugboat had contacted the tanker in Angolan waters and then led it to Nigeria. The tugboat was a replica of one involved in a pirate attack off Gabon last year, he said.
An SOS raised by another tanker in Angolan waters saying it was under attack from pirates on Friday was also a false alarm, he added.
"The navy and the air force went to the location and did not find any signs of an attack. We want to know if this was a diversion tactic and will remain alert as there may be some forces manoeuvring behind these acts," Alfredo said. (Reporting by Shrikesh Laxmidas in Lisbon and Renee Maltezou in Athens)