Two smuggled orangutans repatriated to Indonesia this week are to be prepared for a return to their home in the jungles of Kalimantan, the organisation charged with their care said Thursday.

The great apes, Don-Don aged two-and-a-half and three-and-a-half year old Dong, arrived back from Vietnam on Monday in a speedy repatriation that environmentalists said sent a strong signal to illegal wildlife traffickers.

Aldrianto Priadjati, executive director of the Borneo Orangutan Survival (BOS) Foundation, said that the pair would remain under quarantine for 30 days before their lessons on surviving in the wild would begin.

DNA-testing would first be required to track the female orangutans' origins to one of three sub-species of the apes found in Indonesia's Kalimantan on the island of Borneo.

It may take three to five years to properly prepare them for a return to the wild after their spate in captivity, Priadjati told AFP.

"It will depend on their progress at the rehabilitation centre… They are not too old, but also not too young," he said of the pair.

"We're trying to make them as well as possible to make sure that the orangutans can regenerate in the forests."

He said Don-Don and Dong were "a little bit skinny" but had already put on a kilogram (2.2 pounds) each in the two weeks they have been back in care after being rescued from a private hotel zoo in Vietnam's Ho Chi Minh City.

Training will be a gradual process of introducing them to other orangutans, adapting their diet and allowing them to learn to use forest items, before moving them to a half-way house and then finally the jungle, Priadjati said.

Workers from the BOS and the non-governmental group Wildlife At Risk had found the orangutans and alerted Vietnamese officials, who confiscated them on July 11.

They were smuggled into the country seven to 12 months ago from Kalimantan and bought for a total of 15,000 dollars.

Orangutans are protected under the Convention on International Trade in Endangered Species but experts say the trade in them continues.

Priadjati said that the pair's quick return — following the return of several from Malaysia a few months ago — provided hope for dozens of other orangutans awaiting repatriation from abroad.

He said preparation for transportation of 48 to 53 of the saffron-haired animals was underway after a nearly three-year delay while one or two were also expected from Saudi Arabia.

Orangutans are the only great ape to be found outside of Africa, and are native to the island of Sumatra in Indonesia and to Borneo.

Experts say only about 27,000 remain in the wild and that populations are fast declining due to deforestation and trafficking.