One of the panda cubs recently born at the National Zoo in Washington, DC has died, the zoo confirmed on Twitter.
The smaller panda cub was being kept alive by zookeepers because its mother, Mei Xiang, refused to nurse it, only nursing its larger twin sibling.
Zoo staff attempted to swap the cubs out for feeding with their mother but had not been successful since Monday.
We are sad to report that the smaller of the two panda cubs has died. We will continue to provide updates on social media w/ #Pandastory.— National Zoo (@NationalZoo) August 26, 2015
The small cub weighed 86 grams when it was born, while the larger cub weighed 138 grams. The 17-year-old Mei Xiang weighs 108kg.
Mei Xiang's first cub, Tai Shan, was born in 2005 and returned to China in 2010. Her second cub, Bao Bao, just celebrated her second birthday.
Giant pandas are one of the most endangered species in the world and are notoriously hard to breed in captivity.
The National Zoo is one of only four zoos in the US to have pandas, which are on loan from China.