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 - The National Zoo in Washington says its adult female panda has had twins.

The first cub was born Saturday at 5:35 pm and the zoo said on Twitter that a second was born at 10:07 pm. The zoo has said both cubs appear healthy.

If the cubs survive, they would be the 17-year-old panda's third and fourth surviving offspring.

Mei Xiang's first cub, Tai Shan, was born in 2005 and returned to China in 2010. Her second cub, Bao Bao, turns 2-years-old on Sunday and still lives at the zoo.

The zoo's chief veterinarian Don Neiffer said earlier in the day after the first cub was born that a second cub was a possibility.

Mei Xiang gave birth after her water broke about an hour beforehand, the zoo said in a statement.

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

Kelly said during a news conference later that zoo officials are 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 of the twins.

Chief veterinarian Don Neiffer said the cub has been 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".

Neiffer said zoo staff will 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."

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 cub or which of the sperm donors is the father will be determined later. He added that no decision had been made about naming the cub.

Giant pandas, one of the world's most endangered species, have 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.