The first panda cub, which was about the size of an adult mouse was born at 5:35 p.m. local time (2135 UTC), the zoo said in a statement. Hours later, at 10:07 p.m., its twin arrived, who the zoo officials said was also healthy.

"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," zoo director Dennis Kelly said soon after the birth of the first cub.

The mother panda's maternity team began preparing for the birth about an hour earlier after seeing Mei Xing's waters break on the zoo's panda cam. They hope to carry out a neonatal exam in the coming days and won't know the cubs' sex until a later date.

Seventeen-year-old Mei Xing was artificially inseminated with frozen semen from a male giant panda in April. The male panda, Hui Hui, lives at the China Conservation and Research Center for the Giant Panda in Sichuan province.

She was also inseminated with fresh semen from the zoo's male giant panda Tian Tian. DNA tests will establish which panda is the cubs' father.

After successfully conceiving, Mei Xing began to show signs of pregnancy in July. Common traits include sleeping more, eating less, building a nest and spending more time in her den.

'Mother and baby' time

It will likely be four months before the cubs will venture from the den and visitors can catch a glimpse of the zoo's newest panda.

Chief veterinarian Don Neiffer said Mei Xiang picked up the first cub soon after giving birth and is being "a great mother."

Neiffer said zoo staff will leave the mother and her cubs 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."

Like its siblings, Bao Bao and Tai Shan, the newborn panda cubs will spend four years at the Washington zoo before returning to China.

According to the World Wildlife Fund, there are fewer than 2,000 pandas now left in the wild.

ksb,shs/ng (AFP, AP, Reuters, dpa)