Chief Executive John Lee said on Tuesday that officials need to make sure that parts of the Zoological and Botanical Gardens are properly sealed off, following the deaths of nine monkeys there.

Officials have been investigating the mysterious deaths, as they held a meeting on Monday chaired by Kevin Yeung, the culture, sports and tourism minister.

Lee told reporters that officials need to focus on several things, and one of them is ensuring the affected area of the zoo doesn't expand further.

"The relevant section has been closed off. Disinfection has been conducted. So we have contained the area," he said before the weekly Executive Council meeting.

"The second step is to control. Apart from the cleaning and disinfection, we have to continue monitoring the situation to ensure that the problem will not spread."

Lee said autopsies, tests and toxicology analyses should be completed as soon as possible so that experts can find out the reasons behind the deaths.

Workers need to wear protective gear properly, Lee added.

The chief executive said Yeung is in charge of the issue, and officials will provide an update once there is a new development.