The baby pandas were born Saturday at Adventure World in Wakayama, 280 miles southwest of Tokyo. Their gender has not been confirmed, spokesman Tadashi Ishikawa said.

Although one of the twins weighs just 3 ounces, considered to be premature, both babies and Mei Mei were in good health, Ishikawa said.

Mei Mei, 12, and partner Eimei, 14, are on loan from China.

China's official Xinhua news agency said that the twins' birth raised the number of artificially bred pandas born this year to a record 30.

Ishikawa said, however, that officials could not confirm whether the twins were born as a result of natural mating or artificial insemination.

The panda is one of the world's rarest animals, with about 1,590 living in the wild in China, mostly in Sichuan and the western province of Shaanxi. An additional 180 have been bred in captivity.