Global authorities reacted with alarm on Friday to a new coronavirus variant detected in South Africa, with the EU and Britain among those tightening border controls as scientists 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 World Health Organization (WHO) cautioned against hasty measures.

But European Commission chief Ursula von der Leyen said the EU also aimed to halt air travel from the region.

Scientists are still learning about the variant, identified this week, but the news pummelled global stocks and oil amid fears what new bans would do to already shaky economies across southern Africa.

The variant has a spike protein that is dramatically different to the one in the original coronavirus that COVID-19 vaccines are based on, the UK Health Security Agency said, raising fears about how current vaccines, successful against the more familiar Delta variant, 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.

The WHO was holding a meeting in Geneva, with experts discussing the risks the variant, called B.1.1.529, presents, WHO spokesperson Christian Lindmeier said.

For now, it warned against travel curbs.

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

Nearly 100 sequences of the variant have been reported and early analysis shows it has "a large number of mutations" requiring further study, Lindmeier said.

British health minister Sajid Javid said the new variant had probably spread to other countries.

"The sequence of this variant ... was first uploaded by Hong Kong from a case of someone travelling from South Africa," Javid told lawmakers.

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

Israel barred its citizens from travelling to southern Africa as Prime Minister Naftali Bennett said a few cases of the variant had been reported there,

Israel's Ynet news website reported that, according to the Health Ministry, one of those individuals had received a third shot, or booster, of the Pfizer-BioNTech COVID-19 vaccine two months ago. A Health Ministry spokesperson could not confirm the report.

"We are currently on the verge of a state of emergency," Bennett said, according to a statement from his office.

"Our main principle is to act fast, strong and now."

Too late for travel curbs?

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

"I think we have to recognise that 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, which is to convene its advisory National Coronavirus Command Council on Sunday, will speak to British authorities to try to get them to reconsider their ban, the foreign ministry in Pretoria said.

"Our immediate concern is the damage that this decision will cause to both the tourism industries and businesses of both countries," Foreign Minister Naledi Pandor said in a statement.

Britain and other European countries had already been expanding booster vaccinations and tightening curbs as the continent battles a fourth wave of the coronavirus, led by the delta variant, with many reporting record daily rises in cases.

The new wave and discovery of the new variant come 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, and Germany will declare South Africa a virus variant area, a health ministry source said. France suspended all flights from southern Africa for 48 hours. 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 for visitors from South Africa and five other African countries.

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.