WASHINGTON - The smaller of the two giant panda cubs born at the US' Smithsonian National Zoo over the weekend died Wednesday, zoo officials said.
The undersize cub was less than four days old and had been treated with antibiotics and bottle-fed by zookeepers as its mother, Mei Xiang, focused her attention on its larger sibling.
"We are sad to report that the smaller of the two panda cubs has died," the zoo said on Twitter.
The larger cub "appears to be strong, robust, behaving normally and is with mother Mei Xiang," the zoo said.
The death of the cub, whose up-and-down weight since the birth on Saturday had raised concerns among the zoo keepers, occurred shortly after 2pm on Wednesday.
The twins' birth captured international attention as giant pandas are among the world's most endangered species.
Both cubs were still fur-less and about the size of a stick of butter.
The zoo staff had been caring for the smaller cub and were trying to swop the cubs in Mei Xiang's possession every four hours, it said.
Mei Xiang was artificially inseminated with sperm from Hui Hui, a panda in China, and from the National Zoo's Tian Tian.
Zoo officials have said that they did not yet know which insemination was successful, and that it was possible the twins had different fathers.
Giant pandas, native to China, have a very low reproductive rate, especially in captivity.
There are about 300 giant pandas in captivity and roughly 1,600 in the wild.
Pandas give birth to the smallest babies, relative to the size of the mothers, of any mammal that does not lay eggs or carry their young in a pouch.
The pair was the third set of panda twins born in the United States; only one of those sets have survived.