Authorities have been ramping up social-distancing measures and urging the public to limit their movements. Undersecretary for Food and Health Dr Chui Tak-yi asked residents, especially the elderly and those with health conditions, to remain at home as much as possible.

“Please temporarily stop gatherings and meals with friends and families at home or in public if they are unnecessary,” Chui said. “Protect yourself, protect your families.”

The six infected seamen, from the Philippines, India, Russia and Montenegro, arrived aboard six separate cargo ships, which are now anchored off the south of the city near Lamma Island. They were transferred to hospital and three have been discharged after treatment. It is understood medical staff were sent to carry out tests on the remaining 140 crew members, who will emerge from two weeks of quarantine imposed due to the confirmed cases between this Saturday and August 3.

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Professor David Hui Shu-cheong, a respiratory medicine specialist at Chinese University, said keeping the maritime workers on the vessels while they waited out the incubation period of the disease was an appropriate arrangement.

“It is unlikely the ships will become another Diamond Princess cruise liner, as there are many fewer people on board each ship, and they can be spaced out in their quarantine with little interaction,” Hui said.

The Diamond Princess was hit with a mushrooming Covid-19 outbreak that required the lockdown of the vessel in Yokohama, Japan, early this year. More than 700 passengers and crew became infected, including 76 Hongkongers, three of whom died.

But Dr Leung Chi-chiu, chairman of the Hong Kong Medical Association’s advisory committee on communicable diseases, warned an outbreak aboard a vessel would leave hospitals already struggling to accommodate local patients with the prospect of taking on even more cases.

A source familiar with the matter said the government should at least require maritime workers to undergo quarantine. Hong Kong is the only port in Southeast Asia that allows changeover of crew members, making it a potential transmission hotspot.

Foreign seamen are currently granted the status of “medical surveillance”, exempting them from the mandatory quarantine rules most other arrivals must follow.

They are not required, for instance, to wait for their test result at the main processing facility at AsiaWorld-Expo. “By the time we found out the seamen tested positive, they had already got on board,” the source said.

Officials are battling multiple clusters across the city. Four new cases were tied to Tao Heung restaurant in Mong Kok’s Grand Plaza, pushing the total linked to the eatery above 20. The operator Tao Heung Group said it would temporarily shut its more than 100 outlets for deep cleaning from Sunday.

A resident at the Salvation Army Lung Hang Residence for Senior Citizens in Tai Wai tested preliminary positive for the coronavirus, after two employees – a cleaner and a chef – were confirmed infected. Two other care homes for the elderly have also previously reported outbreaks.

Another new case involved a 43-year-old female sergeant who works at police headquarters in Wan Chai, while a 36-year-old officer stationed at the New Territories South Traffic Headquarters in Tsuen Wan tested preliminarily positive for the coronavirus.

A woman who donated blood at a Red Cross centre in Tsz Wan Shan was confirmed infected on Wednesday. Her red blood cells were given to a patient at Princess Margaret Hospital, who will be tested. “According to overseas experience, the risk of having Covid-19 infection via blood donation or transfusion is very low,” said Dr Linda Yu Wai-ling, a chief manager at the Hospital Authority.

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Another pet was confirmed to have the virus and quarantined. Its owner who lives in Sheung Wan was carrying the virus. Hong Kong earlier recorded two dogs and cats as infected with the virus.

His latest research findings showed the movement of most age groups such as children, adults and the elderly had already returned to March levels when the city was under stringent social-distancing rules. He was hopeful the number of new daily cases would peak in two weeks.

To curb the spread of infections, the government has also extended the work-from-home arrangements for civil servants, which had been scheduled to expire on Sunday, for another week to August 2.

Additional reporting by Gary Cheung