TOKYO (Kyodo) -- A pair of giant pandas who have been in Japan for 13 years on loan returned safely to a giant panda reserve in southwestern China's Sichuan Province on Sunday for treatment, according to the Ueno Zoological Gardens.

Ri Ri and his female mate Shin Shin, both aged 19, came to Tokyo's Ueno Zoological Gardens in February 2011. They have recently been receiving medication for age-related high blood pressure, the metropolitan government said.

"It's a little sad, but the feeling of gratitude is greater. We want them to live peacefully in the environment where they were born and raised," said Yutaka Fukuda, the zoo's head.

The pandas left the zoo early Sunday morning to be transported on a China-bound chartered plane, with hundreds of people seeing them off at Narita Airport near Tokyo. They were carried from the zoo in a special cage by truck.

Around 2,000 people, some of whom lined up all night, visited the zoo on Saturday, the final day for public viewing of the pandas. They took photos, called out their names, bid farewell and thanked them, while the pandas munched on bamboo as usual.

The panda couple "brightened Japan amid a dark atmosphere" after the earthquake and tsunami disaster that devastated northeastern Japan in March 2011 and the novel coronavirus pandemic, Yasumasa Tomita, deputy head of the zoo, told reporters on Saturday.

The metropolitan government said that the two pandas were both born in Sichuan Province in 2005. They arrived at the Ueno zoo for purposes such as reproduction research.

The couple's 10-year lease agreement, originally set to end in February 2021, was extended until February 2026 due to the pandemic, but Japan and China agreed it would be better for them to return to their home country for treatment before their health deteriorates.

They are the parents of Xiang Xiang, born in 2017 and returned to China in 2023, as well as twins Xiao Xiao and Lei Lei, born in 2021. The twins will remain at the Ueno zoo, making them the only pandas there.

The Ueno zoo received its first pandas in 1972 from China to commemorate the normalization of bilateral ties with Japan.