An ailing Thai elephant is being prepared for an arduous journey home from Sri Lanka after a diplomatic dispute between the two Asian nations over the creature's alleged mistreatment.

Thai authorities had gifted the 29-year-old Muthu Raja -- also known back in its birthplace as Sak Surin -- to Sri Lanka in 2001.

But they demanded it back last year after allegations it was tortured and neglected while housed at a Buddhist temple in the island nation's south.

The elephant was in pain and covered in abscesses when Sri Lanka's government took back custody of it from the temple in November.

Most of its wounds have since healed while the elephant recuperates in a zoo on Colombo's outskirts but damage to the animal's foot still requires sophisticated hydrotherapy treatment.

"Arrangements have been made to fly the elephant back to Thailand for this type of therapy," veterinarian Madusha Perera, who has been nursing the creature back to health since its rescue, told AFP on Friday.

Sri Lankan Prime Minister Dinesh Gunawardena told parliament this month he had personally conveyed Colombo's regret to the Thai king over the elephant's alleged mistreatment.

"I was able to re-establish trust between the two countries after an audience with their king," Gunawardena said.

Wildlife minister Pavithra Wanniarachchi said Thailand had been "adamant" in its demands for the elephant's return after its ambassador to Colombo visited Muthu Raja at the temple last year and found the creature in poor health.

Four Thai keepers, along with several local counterparts, are training the 4,000-kilo (8,800-pound) Muthu Raja to stand inside a shipping container-sized cage to acclimatise the creature for its expected flight to Chiang Mai on July 1.

'He needs freedom'

Thai environment minister Varawut Silpa-archa would not be drawn on whether Muthu Raja had been mistreated.

"What happened before, we don't know," he told reporters this month. "The most important thing is the health of Sak Surin."

But he said the Thai government had stopped sending elephants abroad and its diplomatic missions were now checking the condition of those already sent overseas.

The Rally for Animal Rights and Environment (RARE), which led a campaign to rescue Muthu Raja from the temple, is unhappy about the animal's looming departure.

"Our wish was that Muthu Raja will be rested and retired in Sri Lanka," RARE executive director Panchali Panapitiya told AFP. "He needs freedom."

But she said she was thankful for the Thai government's intervention and credits it with saving the elephant's life.

"He would be dead by now if the Thai government did not intervene," Panapitiya said.

"Our request to the next Thai prime minister is to keep him chain-free and let him move on his own."