The discovery of the variant has sparked global concern, a wave of travel bans or curbs and a sell-off on financial markets on Friday as investors worried that Omicron could stall a global recovery from the nearly two-year pandemic.

Israel said it would ban the entry of all foreigners into the country and reintroduce counter-terrorism phone-tracking technology to contain the spread of the variant.

The two linked cases of Omicron detected in Britain were connected to travel to southern Africa, British Health Secretary Sajid Javid said.

Johnson laid out measures that included stricter testing rules for people arriving in the country but that stopped short of curbs on social activity other than requiring mask wearing in some settings.

People who had come into contact with people testing positive for a suspected case of Omicron would have to self-isolate for 10 days and the government would tighten the rules on wearing face coverings, Johnson said, adding the steps would be reviewed in three weeks.

The health ministry in the German state of Bavaria also announced two confirmed cases of the variant. The two people entered Germany at Munich airport on November 24, before Germany designated South Africa as a virus-variant area, and were now isolating, said the ministry.

In Italy, the National Health Institute said an Omicron case had been detected in Milan in a person coming from Mozambique.

Czech health authorities also said they were examining a suspected case of the variant in a person who spent time in Namibia.

Omicron, dubbed a "variant of concern" by the World Health Organisation, is potentially more contagious than previous variants of the disease, although experts do not know if it will cause more or less severe COVID-19 compared with other strains.

The variant was first discovered in South Africa and had also since been detected in Belgium, Botswana, Israel and Hong Kong.

Dutch authorities said 61 of about 600 people who arrived in Amsterdam on two flights from South Africa on Friday had tested positive for the coronavirus. Health authorities were carrying out further tests to see if those cases involved the new variant.

One passenger who arrived from South Africa on Friday, Dutch photographer Paula Zimmerman, said she tested negative but was anxious for the days to come.

Financial markets plunged on Friday, especially stocks of airlines and others in the travel sector. Oil prices tumbled by about $US10 a barrel.

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

Although epidemiologists say travel curbs may be too late to stop Omicron circulating globally, many countries, including the United States, Brazil, Canada and European Union nations, announced travel bans or restrictions on southern Africa on Friday.

The US Centers for Disease Control and Prevention and State Department added on Saturday to Washington's previously announced travel restrictions, advising against travel to eight southern African countries.

Japan and Britain said they were extending travel curbs to more African countries, while South Korea, Sri Lanka, Thailand, Oman, Kuwait and Hungary announced new travel restrictions.

South Africa was worried the curbs would hurt tourism and other sectors of its economy, the foreign ministry said on Saturday, adding the government was engaging with countries that have imposed travel bans to persuade them to reconsider.

The new variant has also thrown a spotlight on disparities in how far the world's population is vaccinated. Even as many developed countries are giving third-dose boosters, less than seven per cent of people in low-income countries have received their first COVID-19 shot, according to medical and human rights groups.

Australian Associated Press