The young primate was five months old when he was discovered inside a wooden crate in the cargo section of a Turkish Airlines plane en route from Nigeria to Thailand, and taken in a zoo in the hills outside Istanbul to recover.

Named Zeytin -- Turkish for olive -- he was nursed back to health with the aim of sending him back to Nigeria, where he began his journey, in line with the regulations in the CITES treaty limiting the trade of protected animals.

Following a Nigerian request for his repatriation, Turkey's nature conservation and national parks directorate began the process but stopped it after a DNA test confirmed Zeytin belonged to a species that was not native to Nigeria.

"The DNA test... using whole genome sequencing, revealed Zeytin was a Western lowland gorilla. This scientific evidence showed that Nigeria was not Zeytin's country of origin (which) necessitated a re-evaluation of Zeytin's conservation status," it said.

The Western lowland gorilla is a critically endangered subspecies native to the rain forests of central Africa, whose numbers have plummeted in recent decades because of deforestation, hunting and disease.

"Since Nigeria is not the country of origin, it was decided... to place Zeytin in a zoo in Turkey," it said. Until now, he has been looked after at Polonezkoy Zoo near Istanbul.

Last month Fahrettin Ulu, regional director of Istanbul's Nature Conservation and National Parks directorate, told AFP it was the first time a gorilla had been seized at Istanbul airport.

When he first arrived, Zeytin weighed 9.4 kilograms (21 pounds) but by early September he weighed 16 kg and his height increased from 62.5 to 80 centimetres (2.1 to 2.6 feet), he told AFP.

Zeytin, "who was once a baby, has become a young gorilla", he added.

According to TRAFFIC, a trade in wild species monitoring group, buyers are increasingly looking for baby great apes as pets or for zoos, circuses, shows -- or for social media content, and were increasingly targeted for being "easy to transport".

TRAFFIC said the Nigerian authorities had been expecting Zeytin to return last month, and were to have sent him to an NGO called the Pandrillus Foundation.

There he would have been housed with another young gorilla of the same subspecies before eventually being sent to a habitat country country sanctuary in central Africa, the foundation's director told AFP.