In the harsh desert steppe of far northwestern China five prehistoric-looking Przewalski's horses, once classed as extinct in the wild, emerge from the endless plains.

The horses -- named after a Russian officer and explorer who spotted them around 1880 -- bear a striking resemblance to those depicted in European cave paintings, with short necks, spiky manes and a yellow hue.

They graze calmly on a few strands of straw as the wind whips across the vast, open landscape.

"These ones here, they can be approached. The others will run away as soon as you get within 300 metres (yards) of them," says Sun Zhicheng, an official at the 660,000-hectare (1.6-million-acre) West Lake national nature reserve.

According to the International Union for Conservation of Nature, Przewalski's horses once roamed as far as Western Europe.

But as the centuries passed, climatic change reduced their habitat and the remainder were so widely hunted, mainly for food, that they were classified as extinct in the wild in 1960 -- although a living specimen was later found in Mongolia.

But a few survived in European zoos, and now efforts are under way to reintroduce them to the wild.

The Chinese project near Xihu in Gansu province faces daunting challenges -- freezing winters, sweltering summers and limited supplies of food and water.

According to Chinese legend, Sun says, the animals were discovered two millennia ago by an exiled criminal around the oasis of Dunhuang, a crossroads on the Silk Road.

"A man had been convicted and banished from Dunhuang. While he was walking near a lake he saw one of these horses.

"He made a mannequin and put it on a path the horse would follow. One day he took the place of the mannequin, and he was able to catch the horse to offer it to the emperor.

"The man then lied to the emperor Han Wudi, saying the horse had sprung out of a spring. And he called it a heavenly horse. The emperor loved the horse so much that he wrote a poem about it."

In 1986 China purchased 18 of the horses from the United States, Britain and Germany and has since bred them in captivity, with their numbers growing to more than 70.

Starting in 2010, carefully selected batches have been released into the reserve.

"Now there are 27, 16 females and 11 males," says Sun. "We even registered the birth of a foal in July 2011, a new success in our reintroduction process."

But very few animals can endure an environment as hard and dry as Gansu's desert steppe.

Przewalski's horses require daily access to water that is within a 30-kilometre (20-mile) range and does not freeze in winter. They also need 10 kilograms (22 pounds) of dry food per day, relatively close to the water.

In a region that receives less than four centimetres (1.5 inches) of rain per year, many of these conditions could become problems, says Sun.

The reserve is taking back-up measures to improve the horses' chances.

"We have increased the water supply by expanding 10 wells. At a later point we are thinking of bringing water from the river.

"In winter we have to break the ice so that the horses can drink."

The horses eat grasses and certain plant species, says reserve employee Lu Shengrong, but when vegetation becomes sparse in winter-time, they will be fed dry alfalfa, straw, black beans and corn.

Of the 2,000 or so Przewalski's horses that now exist worldwide, about a quarter are part of efforts to reintroduce them to the wild, says Claudia Feh, a biologist doing similar work in Mongolia, where several hundred have been released.

The worst threat they face, she says, are ordinary horses, which can infect them with disease or crossbreed with them, diluting the gene pool.

"The biggest enemy is the domesticated horse," Feh says.

Przewalski's horses have "a very narrow genetic base" as all living members of the species are descended from just 13 or 14 individuals, she says.

"They are going to disappear genetically if we do not prevent crossbreeding."

For any wild population to survive long-term, or even 50 years, it must be 1,500 strong, Feh says.

"That goal is far away," she adds. "This is a species that is still rather fragile."