Reuters/Washington

A artificially inseminated giant panda took US zoo officials by surprise on Saturday when she gave birth to twins - more than four hours apart.

Mei Xiang, a star tourist draw in Washington's National Zoo, gave birth to her first cub at 5.34 p.m. (2134 GMT) after her water broke about an hour beforehand, zoo officials said.

"All of us are thrilled that Mei Xiang has given birth," said zoo director Dennis Kelly in a statement before the second cub had arrived. "The cub is vulnerable at this tiny size but we know Mei is an excellent mother."

Kelly told a news conference zoo officials were being cautious and "keeping their fingers crossed" after the zoo lost a six-day-old cub in 2012.

"This is still a very fragile time for this cub," he said of the first arrival, which chief veterinarian Don Neiffer said was showing healthy signs, including vocalizing.

Zoo officials said Mei Xiang picked up the cub soon after giving birth and was being "a great mother." Neiffer said zoo staff would leave the mother and her cub alone for as long as possible.

"We're taking a very hands-off approach," he said. "I'm very much in favor of mom and baby having time together."

Then, about four and a half hours later, a second cub arrived. One cub was placed in an incubator in line with protocol when twins are born.

Mei Xiang previously has given birth to two surviving cubs: Tai Shan in 2005 and Bao Bao in 2013.

Mei Xiang was artificially inseminated on April 26 and 27 with frozen sperm from Hui Hui, a panda in China, and fresh sperm from the National Zoo's Tian Tian. Zoo veterinarians first detected evidence of a fetus on an ultrasound on Aug. 19.

Kelly said the gender of the cubs, or which of the sperm donors is the father, will be determined later. He added that no decision had been made about naming the cubs.

Giant pandas, one of the world's most endangered species, are known for the striking black and white markings that lend their eyes special resonance for human admirers.

With a very low reproductive rate, particularly in captivity, their natural home is in a few mountain ranges in central China. There are about 1,600 giant pandas known to be living in the wild and some 300 in captivity, mostly in China.