The World Health Organisation (WHO) said Omicron may spread more quickly than other forms, and preliminary evidence suggested there is an increased risk of reinfection.

Epidemiologists warned travel curbs may be too late to stop Omicron from circulating globally. The new mutations were first discovered in South Africa and have since been detected in Belgium, Botswana, Israel and Hong Kong.

The US will restrict travel from South Africa and neighbouring countries effective Monday, a senior Biden administration official said.

Canada said it was closing its borders to those countries, following bans on flights announced by Britain, the EU and others.

But it could take weeks for scientists to fully understand the variant's mutations and whether existing vaccines and treatments are effective against it.

Omicron is the fifth variant of concern designated by the WHO.