Hong Kong’s oldest zoo is seeking answers in a monkey medical mystery after nine animals died in two days, including three members of a critically endangered species.

Part of the Hong Kong Zoological and Botanical Gardens has been sealed off and disinfected, and experts have been called in to carry out 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 dead animals included 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,” Mr Lee said.

Hong Kong Monkey Deaths
A buff-cheeked gibbon swings in its cage at Hong Kong’s Zoological and Botanical Gardens (AP)

On Monday, Culture, Sports and Tourism Secretary 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 the behaviour and appetite of another De Brazza’s monkey’s had been found to be unusual, requiring further observation.

But it added that all 80 of the other animals in the gardens are in a normal condition.

In a statement on Tuesday night, the Leisure and Cultural Services Department said the condition of the monkey under observation remains stable.

The department is 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 Central district of the financial hub which returned to Chinese rule in 1997.