One of Mei Xiang's tiny newborn panda cubs died Wednesday, four days after its birth after experiencing respiratory difficulty, Washington's National Zoo said.

Chinese panda Mei Xiang gave birth to twin cubs in the evening of Aug. 22. This is a third time that Chinese giant panda gave birth to twins in the United States.

"We are sad to report that the smaller of the two panda cubs has died," @NationalZoo posted on Twitter.

"The larger cub appears to be strong, robust, behaving normally and is with mother Mei Xiang," the zoo said in a press release.

After the twins were born, zoo keepers decided to swap the cubs in Mei Xiang's possession every four hours so each had time to nurse and spend quality time with her.

The panda team had switched the cubs within the past 24 hours. The smaller cub was with Mei Xiang from about 2 p.m. Tuesday through Wednesday morning. However, when the team switched the cubs Wednesday morning, the smaller cub seemed weaker, was displaying possible respiratory problems, and hadn't gained weight. Zoo vets gave the cub antibiotics, respiratory support, formula and fluids, to no avail, the zoo's chief veterinarian, Don Neiffer, recalled at a news conference on Wednesday.

Neiffer said Mei Xiang wasn't showing preference to one cub over another and was reluctant to give up whichever cub was with her any time keepers attempted a swap. When her second cub was born around 10 p.m. Saturday, she had tried to figure out how to hold both, but was unable to.

The panda team is continuing to closely monitor Mei Xiang and the surviving cub around the clock. At its most recent weigh-in, the surviving cub weighed 137.7 grams, the zoo said

"Mei Xiang gave birth to twin cubs in 2013. One of them failed to survive due to the abnormalities. The other grew up healthy, named Baobao. Last week. Baobao celebrated 2-year-old birthday. The older brother of Baobao, Taishan, the first giant panda born in Washington has returned to China.