The World Health Organization (WHO) yesterday designated a new Covid-19 variant detected in South Africa with a large number of mutations as being "of concern," the fifth variant to be given the designation.

The WHO said in a statement that it had assigned the B.1.1.529 variant the Greek letter "Omicron".

Global authorities yesterday reacted with alarm to the new variant, with the EU, Britain and India among those tightening border controls as researchers sought to find out if the mutation was vaccine-resistant.

Hours after Britain banned flights from South Africa and neighbouring countries and asked travellers returning from there to quarantine, the WHO cautioned against hasty measures.

One South African scientist expert labelled London's decision a symptom of "vaccine apartheid", though European Commission chief Ursula von der Leyen said the EU also aimed to halt air travel from the region and several other countries including India, Japan and Israel toughened curbs.

In Washington, top US infectious disease official Anthony Fauci said no decision had been made on a possible US travel ban.

There was no indication that the variant was in the United States, and it was unclear whether it was resistant to current vaccines, he told CNN.

The WHO said it would take weeks to determine how effective vaccines were against the variant, which was first identified this week, but the news pummelled global stocks and oil amid fears what new bans would do to the global travel industry and already shaky economies across southern Africa.

The variant has a spike protein that is dramatically different to the one in the original coronavirus that vaccines are based on, the UK Health Security Agency said, raising fears about how current vaccines will fare.

"As scientists have described, (this is) the most significant variant they've encountered to date," British Transport Secretary Grant Shapps told Sky News.

In Geneva, the WHO - whose experts yesterday discussed the risks that the variant, called B.1.1.529, presents - warned against travel curbs for now.

"At this point, implementing travel measures is being cautioned against," WHO spokesperson Christian Lindmeier told a UN briefing. "The WHO recommends that countries continue to apply a risk-based and scientific approach when implementing (curbs)."

It would take several weeks to determine the variant's transmissibility and the effectiveness of vaccines against it, Lindmeier said, noting that 100 sequences of it had been reported so far.

British Health Minister Sajid Javid said the sequence of the variant was first uploaded by Hong Kong from someone travelling from South Africa.

"Further cases have been identified in South Africa and in Botswana, and it is highly likely that it has now spread to other countries," Javid told lawmakers.

Belgium said yesterday it has detected the first announced case in Europe of the new variant, in an unvaccinated person returning from abroad.

Israel imposed a travel ban covering most of Africa as Prime Minister Naftali Bennett said a few cases had been reported there.

TOO LATE FOR TRAVEL CURBS?

One epidemiologist in Hong Kong said it may be too late to tighten travel curbs.

"Most likely this virus is already in other places. And so if we shut the door now, it's going to be probably too late," said Ben Cowling of the University of Hong Kong.

South Africa will speak to British authorities to try to get them to reconsider their ban, the foreign ministry in Pretoria said.

Richard Lessells, a South Africa-based infectious disease expert involved in detecting variants, also expressed frustration at travel bans, saying the focus should be on getting more people vaccinated in places that have struggled to access sufficient shots.

"This is why we talked about the risk of vaccine apartheid. This virus can evolve in the absence of adequate levels of vaccination," he told Reuters.

In South Africa, about 35% of adults are fully vaccinated, higher than in most other African nations but around half average levels in developed countries.

EU officials holding an emergency meeting yesterday agreed to urge all 27 nations in the bloc to restrict travel from southern Africa.

Discovery of the new variant comes as Europe and the United States enter winter, with more people gathering indoors in the run-up to Christmas, providing a breeding ground for infection.

Italy imposed an entry ban on people who have visited southern African states in the last 14 days, while France suspended flights from southern Africa and Bahrain and Croatia will ban arrivals from some countries.

India issued an advisory to all states to test and screen international travellers from South Africa and other "at risk" countries, while Japan tightened border controls.

The coronavirus has swept the world in the two years since it was first identified in central China, infecting almost 260 million people and killing 5.4 million.