BANGKOK (REUTERS) - Two critically endangered orang utans smuggled into Thailand three years ago were returned to Indonesia on Thursday (Dec 17), where they will undergo rehabilitation before being released into the wild.

Ung Aing and Natalee, both four-year-old Sumatran orang utans, were taken from a wildlife rescue centre in Ratchaburi province to Bangkok's airport, before being put on a flight to Indonesia, where they will initially stay at a rehabilitation centre in Jambi province on Sumatra island.

Before being put on the flight, the pair were fed bananas and green apples, and cleared of having Covid-19 after taking a test, said Mr Suraphong Chaweepak, a director at the Thai division to protect wild fauna and flora.

"This is the fifth repatriation of orang utans back to Indonesia since 2006," Mr Prakit Vongsrivattanakul, an official at Thailand's Department of National Parks, Wildlife and Plant Conservation, said at the airport. A total of 71 orang utans have now been repatriated to Indonesia from Thailand.

The two great apes were seized on the Thai-Malaysian border in 2017 and after the smugglers were prosecuted, Thailand agreed to send them back to Indonesia, according to a joint statement from Thailand's wildlife and conservation ministry and Indonesia's embassy in Bangkok.

Orang utans are poached illegally from forests for food, to obtain infants for the domestic and international pet trade, or for traditional medicine.

There are only estimated to be around 100,000 Bornean orang utans left in the wild, according to the World Wildlife Fund, while only about 7,500 Sumatran orang utans are thought to remain.

In addition to illegal poaching, populations have crashed because of habitat destruction due to large-scale logging and replacement of forests with cash crops such as oil palm.