National Zoo's popular pandas heading to China

They're leaving town -- and for millions of their adoring fans -- you could call it, well, unbearable.

After more than two decades of "panda diplomacy," Washington's popular pandas were being returned to China Wednesday.

The Smithsonian National Zoo's three current pandas, Mei Xiang (May-SHONG), Tian Tian (tee-YEN tee-YEN) and Xiao Qi Ji (SHIAU-chi-ji), were loaded into special crates Wednesday morning for a 19-hour plane ride aboard the FedEx "Giant Panda Express" to Chengdu, China.

They'll be accompanied by animal care experts and 220 pounds of fresh bamboo, their favorite food.

But because they're in crates, the pandas, who've delighted zoogoers with their slow-moving antics over the years, weren't visible to those wanting to say a final farewell.

Mei Xiang and Tian Tian arrived in Washington in 2000 and their fourth cub, Xiao Qi Ji, was born in 2020. Xiao Qi Ji's siblings were sent to China when each of them was 2 or 3 years old, after their births created a national sensation.

The National Zoo has had giant pandas since 1972, when President Richard Nixon was gifted a pair, Hsing-Hsing and Ling-Ling, by Chinese Communist Party Chairman Mao Zedong in exchange for a pair of arctic musk oxen -- goodwill gestures as the countries opened diplomatic relations.

After the original panda pair passed away in 1999, the National Zoo signed a contract with the China Wildlife and Conservation Association (CWCA) -- "The Giant Panda Cooperative Research and Breeding Agreement" -- and welcomed Mei Xiang and Tian Tian the next year under a 10-year contract that since then has been renewed three times.

Since 1984, Chinese wildlife organizations started lending pandas to other countries, instead of gifting them, in the interest of panda conservation.

With the departure of Mei Xiang, Tian Tian, and Xiao Qi Ji for China, the only pandas remaining in the U.S. will be in Zoo Atlanta, whose contract expires at the end of 2024.

Their leaving comes shortly before the White House says President Joe Biden will meet China's President Xi Jinping later this month during an economic summit in San Francisco, amid tensions between the two countries.

The pandas' return was originally scheduled for December, then moved to Nov. 15 -- and it wasn't clear why it was moved up to this week.

"Giant pandas are not political," Pamela Baker-Masson, director of communications at the National Zoo, said in an earlier interview. "We've been doing this for 51 years, we are very close with our Chinese partners, and we work very, very well together. So, it's about that relationship, and it's about how people from not just China, the United States, but from around the world, work together with one goal and one mission."

But when asked in a recent interview if attempts were made to extend the panda contract, Zoo Director Brandie Smith didn't answer directly.

"Our focus is on the giant panda reproduction, and the kind of behaviors associated with that. And so, we knew when the pandas were post-reproductive they'd return to China to live out their golden years in their homelands," she told ABC News. "And so our plan was always to send them to China at this time."

On Wednesday, as the pandas were being taken from the zoo, Xu Xueyuan, Minister for the Embassy of the People's Republic of China, spoke to pool reporters, remarking that the pandas have been "loving care" the last 23 years. She assured that the pandas are in good health before their flight, and that China will continue to work with the United States to research endangered species.

"As a diplomat in Washington, I say to them goodbye and bon voyage, and as a Chinese government official, I say to them, welcome back," she said. "Giant pandas belong to China, giant pandas also belong to the world."

ABC News' Meghan Mistry contributed to this report.