Hong Kong extended a compulsory quarantine by an extra seven days to 21 days for all visitors outside China 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, Reuters reported.

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.

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.