"We let them stay together last night, but Chuang Chuang was having fun teasing his partner while Lin Hui was sexually ready," Sophon Dumnui, head of the state body which supervises the zoo, told reporters.

"Although we have shown him the how-to-mate video several times, Chuang Chuang is probably a little bit too young to mate. His sex organ is too small and short," Sophon said.

He is six and experts say the male pandas are usually ready to mate at seven, he said.

Ever since the pair was "married" in a Chinese ceremony two years ago, Chuang Chuang's sex life has become a matter of public concern.

Zookeepers have put him on a diet to avoid crushing his partner and have made him watch other pandas mating to get him in the mood.

The pair, on loan from China and a major draw at the zoo, have shown little inclination to consummate their union despite being kept in separate enclosures since December in hopes it would make them lust after each other.

China used the video technique with its pandas, which are notoriously difficult to breed in captivity, and succeeded in producing a cub.

Panda experts will inseminate Lin Hui again on Tuesday and freeze the last portion of Chuang Chuang's semen for future use, panda project chief Prasertsak Boontragulpoontawee said.