Two Omicron cases have been detected in Britain, while Germany and the Czech Republic are investigating probable infections as the world scrambles to curb the spread of the new coronavirus variant.

Britain's health minister confirmed the country's first cases on Saturday, saying both were connected to travel to southern Africa. 

Cases of the new variant, which was first detected in South Africa, have also been reported in travellers in Belgium, Israel and Hong Kong.

A host of countries, including Australia, Brazil, Canada Iran, Japan, Thailand and the United States, have joined others, including the European Union and the UK, in imposing restrictions on southern African countries in response to warnings over the transmissibility of the new variant — against the advice of the World Health Organization (WHO).

Despite the shutdown of flights, there has been increasing evidence that the variant is already spreading.

Germany announced "several Omicron-typical mutations were found in a traveller returning from South Africa".

Dutch authorities are also checking for the new variant after 61 passengers on two flights from South Africa tested positive for COVID-19.

The planes arrived in the Netherlands from Johannesburg and Cape Town shortly after the Dutch government imposed a ban on flights from southern African nations.

South Africa spoke out against the widespread travel bans and restrictions, saying it was being "punished" for detecting the variant.

"This latest round of travel bans is akin to punishing South Africa for its advanced genomic sequencing and the ability to detect new variants quicker," the Ministry of International Relations and Cooperation said in a statement.

The WHO has named the new variant Omicron, labelling it a variant of concern because of its high number of mutations and some early evidence that it carries a higher degree of infection than other variants.

That means people who contracted COVID-19 and recovered could be subject to catching it again.

It could take weeks to know if current vaccines are less effective against the strain.

'It seems to spread rapidly' 

With so much uncertainty about the Omicron variant and scientists unlikely to flesh out their findings for a few weeks, countries around the world have been taking a safety-first approach, in the knowledge that previous outbreaks of the pandemic have been partly fuelled by lax border policies.

"It seems to spread rapidly," US President Joe Biden said Friday of the new variant.

He was speaking only a day after celebrating the resumption of Thanksgiving gatherings for millions of American families and the sense that normal life was coming back, at least for the vaccinated.

In announcing new travel restrictions, he told reporters" "I've decided that we're going to be cautious."

Nearly two years on since the start of the pandemic that has claimed more than 5 million lives around the world, countries are on high alert.

The variant's swift spread among young people in South Africa alarmed health professionals, even though there was no immediate indication  the variant caused more severe disease.

AstraZeneca scientist cautiously optimistic 

In just two weeks, Omicron has turned a period of low transmission in the country into one of rapid growth.

A number of pharmaceutical firms, including AstraZeneca, Moderna, Novavax and Pfizer, said they had plans in place to adapt their vaccines in light of the emergence of Omicron.

Professor Andrew Pollard, the director of the Oxford Vaccine Group which developed the AstraZeneca vaccine, expressed cautious optimism that existing vaccines could be effective at preventing serious disease from the omicron variant.

He said most of the mutations appear to be in similar regions as those in other variants.

"That tells you that despite those mutations existing in other variants the vaccines have continued to prevent serious disease as we've moved through alpha, beta, gamma and delta," he told BBC radio.

He added that it was "extremely unlikely that a reboot of a pandemic in a vaccinated population like we saw last year is going to happen".

Some experts said the variant's emergence illustrated how rich countries' hoarding of vaccines threatened to prolong the pandemic.

Fewer than 6 per cent of people in Africa have been fully immunised against COVID-19, and millions of health workers and vulnerable populations have yet to receive a single dose.

Those conditions can speed up spread of the virus, offering more opportunities for it to evolve into a dangerous variant.

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AP/ABC