HONG KONG (REUTERS, XINHUA) - Hong Kong extended a compulsory quarantine by an extra seven days to 21 days for all visitors outside China, effective Friday (Dec 25), in stepped-up efforts to prevent a new variant of the novel coronavirus from spreading.

Authorities also banned all people who have stayed in South Africa in the past 21 days from boarding for Hong Kong.

Hong Kong has already banned all flights arriving from the United Kingdom from Monday and the city said on Wednesday two students who returned from Britain were likely to be infected with the new super-virulent strain of Covid-19.

In a statement midnight on Friday, authorities said people who have stayed in places outside China during the 21 days before their arrival have to undergo 21 days of compulsory quarantine in designated quarantine hotels.

"Noting the drastic change of the global pandemic situation with the new virus variant found in more countries, there is a need for the government to introduce resolute measures immediately... to ensure that no case would slip through the net even under very exceptional cases where the incubation period of the virus is longer than 14 days," a government spokesman said.

Hong Kong's Center for Health Protection (CHP) reported 57 new confirmed cases of Covid-19 on Friday, taking its total tally to 8,481.

The new cases included 55 local infections, with 25 cases of unknown origin, according to a CHP press briefing. There were also about 50 preliminary cases.

Currently, 940 Covid-19 patients are being treated in public hospitals and the community treatment facility at AsiaWorld-Expo, and 54 patients are in critical condition.

The latest CHP data showed the total number of Covid-19 deaths rose by one to 136 in Hong Kong.