Washimgton - Giant panda Mei Xiang, a star tourist draw in the US capital, gave birth to a pair of cubs hours apart on Saturday, the National Zoo said.Mei Xiang gave birth to her first cub at 17:34 (21:34 GMT) after her water broke about an hour beforehand, zoo officials said. About four and a half hours later, at 22:07, a second cub arrived and appeared healthy, the zoo said."All of us are thrilled that Mei Xiang has given birth," said zoo director Dennis Kelly in a statement following the first birth. "The cub is vulnerable at this tiny size but we know Mei is an excellent mother," he added.Kelly told a news conference later that zoo officials were being very 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 new arrival, which chief veterinarian Don Neiffer said was showing signs of being healthy, including vocalizing.Zoo officials said Mei Xiang picked up the cub soon after giving birth and is being "a great mother".Before the second birth, Neiffer had said zoo staff would leave the mother and her cub alone for as long as possible unless there are signs of a problem."We're taking a very hands-off approach," he said. "I'm very much in favor of mom and baby having time together," he said.After the second, surprise birth, 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, it said.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.