Anyone who is not a citizen of Australia or their dependents, and who has been in African countries where the Omicron variant has been detected and spread within the past 14 days will not be able to enter Australia," Health Minister Greg Hunt told a press briefing.

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.

In today's Covid-19 update, the Ministry of Health said it was monitoring the Omicron variant.

"This particular strain is in its infancy and as with any emerging developments to do with Covid-19 we are closely watching and monitoring evidence and countries' responses," the Ministry said in its statement.

"We will advise on any potential impacts for New Zealand, noting that we remain in a good position to minimise the impact of any new variants with isolation and routine testing of international arrivals."

The United States will restrict entry to travelers from eight southern African countries, President Joe Biden said on Friday.

The policy does not ban flights or apply to US citizens and lawful US permanent residents, a Biden administration official said.

The restrictions will be effective Monday local time and apply to South Africa, Botswana, Zimbabwe, Namibia, Lesotho, Eswatini, Mozambique and Malawi.

The president told reporters while on a walk in Nantucket that his medical team recommended the ban begin on Monday instead of immediately. A White House official told Reuters the gap was due to the procedural things that had to be done before such a ban could be put in place, including working with transportation authorities and airlines.

Airlines for America, an industry trade group, said it was "in communication with the US government as specifics remain unknown at this time and there are many unanswered questions. Amid this rapidly evolving situation, it is critical that US government decisions regarding international travel restrictions and requirements be rooted in science."

The United States could add countries to the restriction list if the variant spreads, a senior administration official said.

Going further, Canada said it was closing its borders to those countries, following bans on flights announced by Britain, the European Union 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.

"As a precautionary measure until we have more information, I am ordering additional air travel restrictions from South Africa and seven other countries," Biden said in a statement.

"These new restrictions will take effect on November 29. As we move forward, we will continue to be guided by what the science and my medical team advises."

The United States only lifted travel restrictions on South Africa on 8 November.

The worries pummelled financial markets, especially stocks of airlines and others in the travel sector, and oil, which tumbled by about $US10 a barrel.

The Dow Jones Industrial Average closed down 2.5 percent, its worst day since late October 2020, and European stocks had their worst day in 17 months.

Cruise operators Carnival Corp, Royal Caribbean Cruises and Norwegian Cruise Line plunged more than 10 percent each, while shares in United Airlines, Delta Air Lines and American Airlines slumped almost as much.

Biden also called on nations meeting at the World Trade Organisation next week to agree to waive intellectual property protections for Covid-19 vaccines in the wake of the discovery of the new variant in South Africa.

"The news about this new variant should make clearer than ever why this pandemic will not end until we have global vaccinations," Biden said.

European Union spokesperson Eric Mamer said member states agreed to the ban after Europe's first case of the variant was discovered in Belgium.

Scientists say they still have much to learn about the virus's new mutations and the WHO has said it will take a few weeks to understand the impact of the new variant, as experts work to determine how transmissible it is.

Where have cases been detected so far?

The WHO says so far fewer than 100 sample sequences have been reported. Cases have mainly been confirmed in South Africa, but have also been detected in Hong Kong, Israel, Botswana and Belgium.

UK Health Secretary Sajid Javid said on Friday that the variant is "highly likely" to have spread to other countries.

Most of the cases in South Africa have been from its most populated province, Gauteng, of which Johannesburg is the capital city.

Only about 24 percent of South Africa's population is fully vaccinated, which could spur a rapid spread of cases there, Dr Mike Tildesley, a member of the Scientific Pandemic Influenza Modelling group (Spi-M), told the BBC on Friday.

In Hong Kong, the infections were found in a person who had arrived from South Africa, and a guest in the same quarantine hotel who tested positive a few days later, the Department of Health revealed. Both were fully vaccinated.

Europe has identified its first case of the variant in Belgium. The Guardian reports that the case emerged in an unvaccinated young woman who had recently travelled from Egypt via Turkey and developed mild flu symptoms 11 days later.

Israel's Prime Minister Naftali Bennett said on Friday it is "on the verge of a state of emergency" regarding the new variant, and that he would "act fast, strong and now".

One case was detected in a person who returned from Malawi, according to Israeli media reports quoting the country's health ministry. Another two suspected infections were yet to be confirmed with test results. All three are said to have been fully vaccinated.

What are different countries doing to stop the spread?

The WHO has warned against countries hastily imposing travel restrictions, saying they should look to a "risk-based and scientific approach."

Nevertheless, an increasing number of nations including the UK and the Netherlands have temporarily halted flights from South Africa, Botswana, Namibia, Zimbabwe, Eswatini (formerly Swaziland) and Lesotho.

The Head of the European Commission, Ursula von der Leyen, has called for all EU nations to put on the "emergency brake" and stop flights from the region.

"It is now important that all of us in Europe act very swiftly, decisively and united," she said on Friday.

The EU is already struggling to cope with fresh waves of the virus, and several countries have re-imposed lockdowns and restrictions.

Singapore, Italy, France and Israel are among those who have also placed Mozambique on their red lists.

The Czech Republic will ban anyone who has spent more than 12 hours in those nations and also Zambia from entering as of Saturday.

Japan has announced that from Saturday, travellers from much of southern Africa will need to quarantine for 10 days and take a total of four tests during that time.

"The key to crisis management is to prepare for the worst," the Japan Times newspaper quoted chief cabinet secretary Hirokazu Matsuno as saying.

Germany, which is currently going through a fourth wave of infections because of the Delta variant, will allow only Germans to fly into Germany from South Africa as of Friday night. The head of the country's RKI public health institute, Lothar Wieler, said he was very worried about the new variant.

Meanwhile, the German air force on Friday began airlifting patients within the country for the first time during the pandemic, flying people from Bavaria to Münster-Osnabrück in the west, where there are more intensive care beds free.

India has ordered more rigourous screening and testing for travellers arriving from South Africa, Botswana and Hong Kong, local media is reporting.

Reactions from South Africa

Health Minister Joe Phaahla told reporters that the flight bans against the country were "unjustified".

"The reaction of some of the countries, in terms of imposing travel bans, and such measures, are completely against the norms and standards as guided by the World Health Organisation," Phaahla said.

The bans already appear to be causing confusion at a number of airports. Passengers on a plane travelling from Cape Town to Manchester via Amsterdam told the BBC that they were held on the tarmac at Schiphol Airport in the Dutch capital for four hours, before eventually disembarking to take PCR tests.

Meanwhile, the flight bans threaten to separate families over the festive season.

Ruth Daines-Slack, 70, had booked to fly from Cape Town to the UK next week for Christmas and her mother's 100th birthday celebration. She told the BBC she is "beyond devastated" that she will no longer be able to make the trip, adding that the UK's travel ban is a "knee-jerk reaction".

The UK was one of the first nations to impose a flight ban, which South Africa's foreign ministry has criticised as a "rushed decision" made before the WHO has made any announcement on the variant.