BERLIN (AP) — Australia on Sunday became the latest country to detect the omicron variant of the coronavirus in travelers who arrived from southern Africa, while Israel decided to bar entry to foreign nationals — the toughest of a growing raft of curbs imposed by nations around the world as they scramble to slow its spread.

Confirmed or suspected cases of the new variant have already emerged in several European countries, in Israel and in Hong Kong, just days after it was identified by researchers in South Africa.

The “act first, ask questions later” approach reflected growing alarm about the emergence of a potentially more contagious variant nearly two years into a pandemic that has killed more than 5 million people, upended lives and disrupted economies across the globe.

 The Dutch public health authority confirmed Sunday that 13 people who arrived in the Netherlands on flights from South Africa on Friday have so far tested positive for the new omicron coronavirus variant.

The 61 people who tested positive for the virus on Friday after arriving on the last two flights to Amsterdam’s Schiphol airport before a flight ban was put in place were immediately put into isolation while sequencing was carried out to establish if they had the new variant.

While much remains to be learned about the new variant, researchers are concerned that it may be more resistant to the protection provided by vaccines and could mean that the pandemic lasts for longer than anticipated.

Israel moved to ban entry by foreigners and mandate quarantine for all Israelis arriving from abroad.

“Restrictions on the country’s borders is not an easy step, but it’s a temporary and necessary step,” Prime Minister Naftali Bennett said at the start of the weekly Cabinet meeting.

Dr. Ran Balicer, head of the government’s advisory panel on COVID-19, told Israel’s Kan public radio that the new measures were necessary for the “fog of war” surrounding the new variant, saying it was “better to act early and strictly” to prevent its spread.

Many countries have restricted or banned travel from various southern African countries — among the latest New Zealand, Thailand, Indonesia, Singapore, Sri Lanka, the Maldives and Saudi Arabia. Places that already had imposed restrictions include Brazil, Canada, the European Union, Iran, and the U.S. This goes against the advice of the World Health Organization, which has warned against any overreaction before the variant is thoroughly studied.

Authorities in Australia said two overseas travelers who arrived in Sydney from Africa became the first in the country to test positive for the omicron variant. Arrivals from nine African countries are now required to quarantine in a hotel upon arrival.

The United States’ top infectious diseases expert, Dr. Anthony Fauci, said he would not be surprised if the omicron variant was already in the U.S., too.

“We have not detected it yet, but when you have a virus that is showing this degree of transmissibility … it almost invariably is ultimately going to go essentially all over,” Fauci said on NBC television.

In Europe, much of which already has been struggling with a sharp increase in cases over recent weeks, officials also were on their guard.

The U.K. on Saturday tightened up rules on mask-wearing and on testing of international arrivals after finding two omicron cases. Spain announced it won’t admit unvaccinated British visitors starting Dec. 1. They are currently allowed to enter with a negative coronavirus test.

Italy was going through lists of airline passengers who arrived in the past two weeks after a business traveler who returned from Mozambique and landed in Rome on Nov. 11 tested positive for omicron.

“The phase of searching for the new variant has started. Controls at airports, ports and train stations have been reinforced,’’ said the Lazio region’s top health official, Alessio D’Amato. The region that includes Rome’s Leonardo da Vinci international airport also is sending random virus test samples to the Spallanzani infectious disease hospital in Rome to be analyzed for the new variant.

In France, Health Minister Olivier Veran said that while no cases have yet been confirmed in France, “it’s a question of hours,” given that omicron infections have been reported in multiple neighboring countries. “It is probable that there currently are cases in circulation,” he said on a visit to a Paris vaccination center.

While it is not clear yet how existing vaccines work against the omicron variant, Veran said the French government isn’t changing its strategy to fight the latest surge of infections driven by the delta variant, which centers on increasing vaccinations and boosters.

David Hui, a respiratory medicine expert and government adviser on the pandemic in Hong Kong, said that even though it is not clear if current coronavirus vaccines are effective against the new variant, the city’s vaccination rate should be increased and booster doses should be implemented as soon as possible.

He said that the two people who tested positive for the omicron variant had received the BioNTech-Pfizer shot and exhibited very mild symptoms, such as a sore throat.

“Vaccines should work but there would be some reduction in effectiveness,” he said.

Zen Soo reported from Hong Kong. Adam Schreck in Bangkok and Associated Press writers around the world contributed to this report.