China's artificial breeding program for the country's iconic panda has yielded an extremely rare event, the arrival of panda triplets, officials announced.

The mother, Ju Xiao, gave birth to the triplets in the city of Guangzhou at the end of July, but the announcement was delayed until it was evident all three of the cubs would survive, experts at the city's Chimelong safari park said.

The event is a milestone given the consistently low reproductive rate of the species, they said.

"It was a miracle for us and [the births] exceeded our expectations," park manager Dong Guixin said. "It's been 15 days. They have lived longer than any other triplets so far."

The triplets were conceived naturally when 12-year-old Ju Xiao was paired with 17-year-old Linlin, another panda in residence at the park.

Ju Xiao stayed under around-the-clock observation as the birth approached.

The birth of the triplets took four hours, leaving the mother too exhausted to care for them after they were born.

The three cubs were placed in incubators, then returned to the mother after she gained her strength and was able to nurse them, park workers said.

A team of feeders was also on hand to help, they said.

The panda cubs currently weigh between 2.9 and 4.4 ounces, having been smaller than a human hand at their birth.

Although they are the only knows panda triplets alive -- and only the fourth ever born in breeding programs -- they are too young to be officially considered as surviving since mortality rates among newborn pandas is high, said an official at China's Sichuan Wolong national nature reserve, universally considered the world's foremost panda research center.

"We can only say they are surviving once they reach six months. For now they are indeed the only surviving triplets," the center official said.

It's the first litter of three cubs known to have survived more than 15 days. Of triplets born in 1999 and 2013, only one survived. In another triplet birth in 1967, none of the cubs survived.

Due to their low birth rates and pressures of loss of habitat and human encroachment, pandas are considered critically endangered, with just around 1,600 in the wild, mostly in the mountainous regions of the southern Chinese province of Szechuan.

There are more than 300 living in captivity, most in the country's breeding programs, although there are also pandas on loan to zoos around the world.