Eight of the animals were found dead on Sunday and another died on Monday after displaying unusual behaviour at Hong Kong Zoological and Botanical Gardens.

Parts of the zoo have been sealed off and disinfected and experts have been called in to conduct necropsies and toxicological tests, Hong Kong leader John Lee said.

The dead animals included a De Brazza's monkey, a common squirrel monkey, four white-faced sakis and three cotton-top tamarins, which are a species listed as critically endangered by the International Union for Conservation of Nature.

Zoo staff are also monitoring a De Brazza's monkey that was behaving unusually, the government said.

All 80 other animals in the gardens were in a normal condition, it added.

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The deaths raised concerns about a possible outbreak of a zoonotic disease such as monkeypox, which can jump from animals to humans, said Jason Baker, senior vice president of animal rights group PETA Asia.

"Monkeys in captivity are often exposed to pathogens that cause diseases that can be transmitted to humans, including tuberculosis, Chagas disease, cholera and MRSA," he said.

He argued the only way to ensure the well-being of animals and prevent the spread of such diseases was to stop confining them in unnatural environments.