A handout picture made available by the Smithsonian's National Zoo on 23 August 2015 shows veterinarians examining one of the two new born giant panda cubs at the National Zoo in Washington, USA, August 22, 2015. [Photo/IC]

"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.