* Tanker was about to offload fuel in Abidjan when seized
* Ghana denies locating ship, current position unclear (Adds quotes, detail)
By Joe Bavier
ABIDJAN, Jan 21 (Reuters) - Gunmen seized a Nigerian-owned, Panama-flagged tanker with 16 Nigerian crew off Ivory Coast's port of Abidjan as it prepared to unload 5,000 tonnes of fuel, port officials said on Monday.
Attacks on shipping are increasing in the Gulf of Guinea - second only to the waters around Somalia for piracy. But the ITRI incident was only the second of its kind in Ivorian waters.
The tanker, named the ITRI and owned by Lagos-based Brila Energy, was commandeered on Thursday, Abidjan's port authority said in a statement.
Serge Constant of Koda Maritime, an Ivorian firm that was managing its stopover in Ivory Coast, said there has been no contact with it since.
Constant said the ITRI's onboard tracking system had been disabled. Abidjan port officials said the ITRI's last known position was off the coast of neighbouring Ghana. But Ghanaian authorities said they had been unable to locate the ITRI.
"...We now seem to be back to square one. The information is contradictory. We don't know who's telling us the truth and who isn't," said Constant.
Piracy subsided to a five-year low in 2012 due mainly to a drastic reduction in Somali hijackings in the seas off the Horn of Africa. But 10 vessels with a total of 207 crew were seized in the Gulf of Guinea off West Africa last year, according to the International Maritime Bureau.
Many of the pirate gangs in the Gulf of Guinea are offshoots of militant groups that once operated in Nigeria's oil-rich Niger Delta before they agreed an amnesty in 2009.
Analysts say coordinated efforts by Nigerian authorities and neighbouring countries have forced Nigerian pirates to seek easier targets outside their home waters.
"Nigeria and Benin have had joint actions for two years, and they have been quite successful. We haven't seen the kinds of heavy attacks that we used to see," said Martin Ewi, a senior researcher with South Africa's Institute for Security Studies.
"Ivory Coast seems to be attracting those that have been driven out."
In October, suspected Nigerian pirates seized a Bahamas-flagged tanker carrying more than 32,000 metric tonnes of gasoline near Abidjan's port. The 24 crew were later freed unharmed. (Reporting By Joe Bavier; Additional reporting by Kwasi Kpodo in Accra; Editing by Richard Valdmanis and Mark Heinrich)