The National Zoo's giant panda Mei Xiang is shown in this Giant Panda Cam image released in Washington, DC August 22, 2015. Giant panda Mei Xiang gave birth to a cub at the Smithsonian?s National Zoo Saturday, August 22, 2015. [Photo/Agencies]

WASHINGTON - 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 5:34 pm (2134 GMT) after her water broke about an hour beforehand, zoo officials said. About four and one-half hours later, at 10:07 pm, 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.