The U.K., Australia and Italy have detected the Omicron variant of the coronavirus, as other nations investigated suspected cases of a strain that health authorities say could be more transmissible and has been driving a jump in infections in South Africa.

The British government tightened restrictions, including imposing new mask mandates and PCR tests for travelers, after identifying the first two cases of the variant in the country, connected to South African travel. The individuals have been told to self-isolate along with...

The U.K., Australia and Italy have detected the Omicron variant of the coronavirus, as other nations investigated suspected cases of a strain that health authorities say could be more transmissible and has been driving a jump in infections in South Africa.

The British government tightened restrictions, including imposing new mask mandates and PCR tests for travelers, after identifying the first two cases of the variant in the country, connected to South African travel. The individuals have been told to self-isolate along with their households.

Two travelers who arrived in Sydney, Australia, from southern Africa on Saturday were found to be infected with the variant, state health authorities there said Sunday. Both were reported as being asymptomatic and fully vaccinated.

Italy identified its first case of omicron variant from a traveler arriving from Mozambique, according to Italy’s National Health Institute.

Dozens of governments around the globe have restricted travel from South Africa and neighboring countries to try to avoid the variant’s spread. The World Health Organization on Friday declared Omicron a “variant of concern,” indicating that it carries higher risks than other virus strains.

Health authorities in Belgium, Israel, Hong Kong and Botswana have already detected cases of the variant.

Earlier Saturday, Dutch authorities said 61 of 600 passengers arriving from South Africa on two KLM flights tested positive for the coronavirus. Health authorities are still analyzing whether those passengers carried the new strain. Passengers who tested positive for Covid-19 will be placed in isolation at a hotel.

Passengers aboard a KLM flight waited on the tarmac at Schiphol airport in Amsterdam after it landed from South Africa on Friday.

Photo: /Associated Press

In Germany, authorities in the state of Hesse, home to Frankfurt International Airport, said Saturday that they had identified several mutations present in the Omicron variant in samples taken from a traveler returning from South Africa. Two more suspected cases have been found in the state of Bavaria, the regional health ministry said, in people who returned from South Africa on Nov. 24. They are isolated at home, the ministry said.

President Biden has been briefed on the latest and senior health officials and the Covid response team are in touch with health officials around the world, the White House said. In preparation for a potential Covid-19 surge, New York’s governor declared a state of emergency, which would allow overwhelmed hospitals to turn away patients seeking nonurgent or nonessential health procedures.

The WHO said preliminary evidence indicated that the strain could pose greater risk of people falling ill with Covid-19 a second time and that it could be more transmissible than other strains of the coronavirus, citing Omicron’s rapid spread in South Africa.

In its statement Friday, the WHO didn’t mention Omicron’s potential impact on the effectiveness of Covid-19 vaccines. But several scientists have said that among Omicron’s around 50 mutations—compared with the original coronavirus detected in Wuhan, China, in 2019—there were several that in other variants allowed the virus to evade parts of the immune response triggered by vaccination.

Epidemiologists are divided on whether travel bans are an effective way to stop new variants from moving to other countries. Some have said that Omicron’s early detection and South Africa’s decision to rapidly alert the world to its potential risks have given health authorities elsewhere a chance to at least slow down its spread.

But the high number of passengers testing positive on the two Dutch flights highlights how difficult that may be. Flights from South Africa to the Netherlands take around 11 hours. Passengers are required to wear masks, but they can be removed for meals and beverages. The passengers on the two Dutch planes were also held for hours on the tarmac and then taken for testing, likely creating opportunities for healthy passengers to catch the virus from those already infected.

About 400 of the 600 passengers who arrived at Amsterdam’s Schiphol Airport on Friday were connecting to other destinations in KLM’s network, according to a spokesman for the airline.

A health worker conducted a Covid-19 test at Johannesburg’s OR Tambo International Airport on Saturday.

Photo: phill magakoe/Agence France-Presse/Getty Images

Both KLM flights from South Africa arrived before the new restrictions—which include evidence of a negative PCR test—came into effect at 12 p.m. on Friday. Vaccinated passengers weren’t required to provide evidence of a negative test before boarding the flight. The Dutch government opted to test the incoming passengers as a precaution, according to the spokesman.

KLM said on Friday it would continue to operate flights to South Africa in line with the new government mandates.

The British government added Malawi, Mozambique, Zambia and Angola to the list of countries on its “red list,” to which travel restrictions apply. Six countries, including South Africa, Namibia and Zimbabwe, were put on the list on Friday.

Concern over the new variant comes as European countries are tightening rules to try to curb a surge in coronavirus cases.

British Prime Minister Boris Johnson announced new restrictions, including compulsory wearing of face-masks on public transport and in stores. For people entering the U.K., it will now be mandatory to take a PCR test on the second day after arrival and isolate until they receive a negative result.

All contacts of those who test positive for the Omicron variant will have to isolate for 10 days regardless of vaccination status. Mr. Johnson said the restrictions would buy time to accelerate and expand the U.K.’s vaccine booster program, with a target of 6 million shots in the next three weeks.

The moves were driven by what Chief Medical Officer Chris Whitty described as “strong theoretical reasons to believe that some degree of vaccine escape is likely given the number of mutations.”

The Dutch government announced new restrictions on Friday, including a closure of all but essential stores and a ban on amateur sports matches and training from 5 p.m. to 5 a.m.

Two cases of the Omicron variant detected in Hong Kong, which has some of the strictest quarantine requirements in the world, also show how hard it may be to contain the new strain. The city’s health department said in a Thursday statement that samples taken from the two people infected with Omicron suggested that one had caught the virus from the other, who had recently returned from South Africa.

Health workers in the Netherlands transported passengers who tested positive for Covid-19 to a hotel for quarantine.

Photo: laurens bosch/Shutterstock

Since its detection earlier this month, Omicron has driven a sharp increase in South African cases, from around 300 just over two weeks ago to 2,828 on Friday, quickly crowding out the Delta variant that is currently dominant globally.

South African officials were deliberating Saturday whether to place new restrictions on social and economic life in the country, which has already been hit hard by the pandemic.

Excess deaths—which experts say provide a better picture of the virus’s toll on South Africa than the official number of Covid-19 deaths because of low testing rates—have topped 272,000 since the start of the pandemic, one of the highest per capita counts in the world.

The international arrivals area at London's Heathrow Airport in the U.K., which has restricted travel from South Africa and several neighboring countries.

Photo: Alberto Pezzali/Associated Press

—Natasha Khan contributed to this article.

Corrections & Amplifications
Chelmsford is about 40 miles northeast of London. An earlier version of this article incorrectly said it was to the northwest. (Corrected on Nov. 27)

Write to James Marson at james.marson@wsj.com, Gabriele Steinhauser at gabriele.steinhauser@wsj.com and Benjamin Katz at ben.katz@wsj.com