Photo used for illustrative purpose.

The lengthy lockdown order — reminiscent of those used on the Chinese mainland — comes as Hong Kong tacks harder to Beijing's "zero-Covid" policy.

At least 20 confirmed or preliminary positive cases were found in a public housing block in the Kwai Chung neighbourhood, said health chief Sophia Chan. "Clearly there is a community outbreak and the situation is worrying," Chan told reporters.

A security guard, cleaner and multiple residents living in non-adjacent flats have been infected, with the source thought to be a man who visited the building on Jan.13.

"These infection clusters involve superspreading and are of extremely high risk," Chan added.

Edwin Tsui, who heads the Centre for Health Protection, said the virus might have been spread by a cleaner who had to work floor by floor. Some 2,700 residents will need to stay home until Jan.26 and be tested daily.

Like China, Hong Kong stamps out even the slightest trace of the virus with largely closed borders, contact tracing, targeted lockdowns and long quarantines.

The five-day "snap lockdown" is the longest ordered yet in the city. Previous building lockdowns have tended to be overnight with residents allowed to leave once they show a negative test.

Those who test positive are taken to isolation wards, while close contacts go to a government quarantine camp.

The Hong Kong government has said it will provide food and anti-epidemic supplies for affected residents.  But some people were spotted buying groceries in bulk on Friday afternoon, according to local media reports.

Hong Kong's strict rules have kept the city largely virus-free but internationally isolated. Already strict social-distancing rules were ramped up after an Omicron outbreak began last month.

Agence France-Presse