The smaller of the two giant panda cubs born at the US`s 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.
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.