Hong Kong extended a compulsory quarantine by an extra seven days to 21 days for all visitors outside China, effective Friday, 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 the UK 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.