Online sales website Gumtree said Monday that it had banned advertisements offering donkeys in South Africa to prevent inhumane killings of the animals to meet Chinese demand for their skins.

Donkeys are being increasingly slaughtered in South Africa to supply smugglers who export the skins to China, where they are believed to have medicinal properties.

"Gumtree has decided to ban the sale of all donkeys and mules on the site, not just the hides and byproducts," the company said.

"We hope that this will play a small part in reducing animal cruelty."

Gelatin-rich donkey skin is used in traditional Chinese medicine to treat ailments such as insomnia, and is also an ingredient in a treatment said to delay menopause in women.

Donkey meat is also eaten in some parts of China.

South African police say there has been a surge in the illegal slaughter of the animals since last year, with thousands of hides found hidden in warehouses and backyards.

The illicit trade has also extended to neighbouring countries, including Botswana and Namibia.

According to the South Africa's National Council of Society for the Prevention of Cruelty to Animals, the donkeys are stolen from owners and often bludgeoned to death with hammers or even skinned alive.

The organisation welcomed the move by Gumtree, saying it would help raise awareness of the slaughter.

"It is a meaningful step in the prevention of cruelty to animals and the protection of them by removing a specific market or forum for those who trade in them or their parts," it said.

No donkeys appeared to be for sale on Gumtree late Monday.