URUMQI, May 23 (Xinhua) -- Four foals of Przewalski's horse, a type of endangered horse, have been born since the breeding season starts in May, a breeding center in northwest China's Xinjiang Uygur Autonomous Region said Thursday.

Przewalski's horses are the only surviving horse subspecies never to have been domesticated. They have historically lived on grasslands that are now part of China's Xinjiang and Mongolia.

The wild horse breeding research center at the Kalamaili State Nature Reserve in northern Xinjiang is the largest center of its kind in Asia. It has worked toward restoring the population of wild horses, said Zhang Hefan, the center's senior engineer.

Since 2001, 110 horses have been released to the wild, he said.

So far, the nature reserve has 92 captive bred horses, 102 in the semi-wild situation and 224 in the wild.

"Newborn horses usually come at night or in the morning, so we have to be on alert," said Tursun, a breeder.

The Przewalski horses faced extinction in the wake of rampant hunting beginning in the early 19th century.

China started a breeding program in 1986 using horses brought back from Britain and Germany to repopulate the subspecies. Worldwide, about 2,000 Przewalski's horses remain.