The Netherlands confirmed 13 cases of the new omicron variant of the coronavirus on Sunday and Australia found two as the countries half a world apart became the latest to detect it in travelers arriving from southern Africa.

Israel decided to bar entry to foreigners and Morocco said it would suspend all incoming air travel from around the world for two weeks starting Monday — the most drastic of a growing raft of travel curbs being imposed by nations around the world as they scramble to slow the variant's 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.

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.

The Dutch public health authority confirmed that 13 people who arrived from South Africa on Friday have so far tested positive for omicron. They were among 61 people who tested positive for the virus after arriving on the last two flights to Amsterdam's Schiphol airport before a flight ban was implemented. They were immediately put into isolation, most at a nearby hotel, while sequencing was carried out.

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. Two German states reported a total of three cases in returning travelers over the weekend.

Read: South African scientists brace for wave propelled by omicron

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.

Morocco's Foreign Ministry tweeted Sunday that all incoming air travel to the North African country would be suspended to "preserve the achievements realized by Morocco in the fight against the pandemic, and to protect the health of citizens." Morocco has been at the forefront of vaccinations in Africa, and kept its borders closed for months in 2020 because of the pandemic.

Dutch Health Minister Hugo de Jonge said he asked his country's public health institute for advice on whether additional travel restrictions are needed, but he wants to coordinate with his European Union counterparts because "I think those are really steps that we will have to take together."

Many countries have restricted or banned travel from various southern African countries — among the latest New Zealand, Thailand, Indonesia, Singapore, Sri Lanka, the Maldives, Saudi Arabia, Qatar and the United Arab Emirates. Places that already had imposed restrictions include Brazil, Canada, the EU, 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.

South Africa's government responded angrily to the travel bans, which it said are "akin to punishing South Africa for its advanced genomic sequencing and the ability to detect new variants quicker." It said it will try to persuade countries that imposed them to reconsider.

"Whilst we respect the right of all countries to take the necessary precautionary measures to protect their citizens, we need to remember that this pandemic requires collaboration and sharing of expertise," the minister for international relations and cooperation, Naledi Pandor, said in a statement.

Read: Britain tightens COVID rules as world on alert over omicron

The United States' top infectious diseases expert, Dr. Anthony Fauci, said he wouldn't be surprised if the omicron variant was already in the U.S., though it hasn't yet been detected there.

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

The U.K. on Saturday tightened 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.

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 Lazio region's top health official, Alessio D'Amato, said that "controls at airports, ports and train stations have been reinforced."

French Health Minister Olivier Veran said that, while his country had no confirmed cases yet, "it is probable that there currently are cases in circulation."

While it is not clear yet how existing vaccines work against the omicron variant, Veran said France 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 the two people who tested positive for the omicron variant had received the 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.