Part of the Hong Kong Zoological and Botanical Gardens has been sealed off and disinfected, and experts have been called in to conduct necropsies and toxicological tests, Hong Kong leader John Lee said in his weekly press briefing on Tuesday.
Eight monkeys were found dead on Sunday and another died on Monday after displaying unusual behaviour.
The deceased animals are a De Brazza's monkey, a common squirrel monkey, four white-faced sakis and three cotton-top tamarins - a species listed as critically endangered by the International Union for Conservation of Nature.
"Whenever we have any news, if there's a new development, an announcement should be made as soon as possible, so that everyone can know about the facts," Lee said.
On Monday, Secretary for Culture, Sports and Tourism Kevin Yeung held an urgent interdepartmental meeting about the deaths with the Leisure and Cultural Services Department, the Agriculture, Fisheries and Conservation Department and the Department of Health.
It said in a statement that another De Brazza's monkey's behaviour and appetite were found to be unusual, requiring further observation.
All 80 other animals in the gardens were in normal condition, it added.
In a statement on Tuesday night, the Leisure and Cultural Services Department said the monkey's condition remained stable.
The department was still awaiting test results and provided no clues about the possible cause of the deaths.
The Hong Kong Zoological and Botanical Gardens - the oldest park in the former British colony - fully opened to the public in 1871.
It is a rare urban oasis in the downtown central district of the financial hub which returned to Chinese rule in 1997.
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