Countries in the Asia-Pacific region are creating tougher rules for those infected with the Omicron Covid-19 variant, as governments move more aggressively than they have against all other previous iterations, including Delta.

The modifications come as Omicron begins to reach the Asia-Pacific region, with Australia, Hong Kong, India, Japan, South Korea, Singapore and Malaysia reporting confirmed cases. The initial moves focused on national travel restrictions and border tightenings.

Scientists...

Countries in the Asia-Pacific region are creating tougher rules for those infected with the Omicron Covid-19 variant, as governments move more aggressively than they have against all other previous iterations, including Delta.

The modifications come as Omicron begins to reach the Asia-Pacific region, with Australia, Hong Kong, India, Japan, South Korea, Singapore and Malaysia reporting confirmed cases. The initial moves focused on national travel restrictions and border tightenings.

Scientists are still racing to understand Omicron, but in an acknowledgment of the potential threat, countries are going beyond their previous variant-containment efforts.

In South Korea, Japan and Singapore, officials have chosen to move those infected by the Omicron variant into hospitals or other government-run facilities under medical supervision, even if the individuals are asymptomatic or experiencing mild symptoms. Until recently, patients have been allowed to quarantine at home.

“We are operating under the premise that the Omicron variant poses a grave danger,” a South Korean senior health official, Son Yong-rae, said Thursday. Officials said there was a high possibility the Omicron variant was already spreading across the country.

Japan is taking extra Omicron measures. It has designated everyone on two flights with confirmed Omicron cases as close contacts, urging them to move to government-designated facilities and get tested every two days. Previously, only passengers two rows to the front or the back of confirmed cases were considered close contacts—and those people could quarantine at home and had to be tested just once.

The CDC said the first known U.S. case of the Omicron variant has been identified in California. The coronavirus strain is spreading across the world as scientists race to find out more about its effects. WSJ’s Brianna Abbott explains what could be next for the U.S. Photo: Spencer Platt/Getty Images The Wall Street Journal Interactive Edition

Passengers aren’t legally required to stay in the government facilities, but Japanese officials have warned they would release the names of people who decided to stay at home for self-isolation but who break quarantine rules. Health officials are checking in with close contacts by phone, rather than using a chatbot.

“This is an extraordinary handling,” as part of stepped-up measures against the Omicron variant, said a health ministry official who briefed reporters on Japan’s first Omicron case on Tuesday.

The new rules for those infected are layered on top of a host of new travel restrictions in the region. Japan has closed the door to all nonresident foreigners. In Hong Kong, even transit passengers will now be required to do a Covid-19 test 72 hours before boarding, the city said Thursday. People traveling to other destinations through Hong Kong weren’t previously required to provide a negative result.

The directive came after a 38-year-old transit passenger spent three days in the Hong Kong airport’s restricted area due to a visa-related issue, before testing positive for the Omicron variant on Nov. 27 during a pre-departure test. The passenger arrived in Hong Kong from Nigeria after flying through Qatar.

Much remains unknown about the Omicron variant. But part of the motivation for treating Omicron cases differently is the early suggestion by the World Health Organization that it can spread more quickly than previous variants.

Quarantine staffers walked through Haneda Airport in Japan earlier this week.

Photo: kazuhiro nogi/Agence France-Presse/Getty Images

Determining where confirmed Omicron patients go—and how wide to cast the net for potential spread—are key variables for health officials who are trying to understand the extent of potential outbreaks.

Various countries’ moves to enforce strict travel restrictions and quarantine measures are an overreaction that won’t accomplish much in the long run, said Kim Dong-hyun, a professor of epidemiology at South Korea’s Hallym University College of Medicine.

“We’re already seeing that the variant has spread regionally,” Dr. Kim said.

South Africa reported the first Omicron case last week and days later, the WHO deemed it a variant of concern. As of Wednesday, the Omicron variant had spread to 23 countries, with the majority in Europe and Africa, according to the WHO’s latest figures. On Friday morning, Australia’s chief medical officer said there were a total of 419 Omicron cases in roughly 30 countries.

Another reason to treat Omicron differently than previous Covid-19 variants is that conducting the genomic sequencing to determine whether a patient has been infected takes longer.

Such testing takes five days, South Korean health officials said. Prior variants, including Delta, can now be identified with PCR tests in just a day, they said.

Beyond treatment and testing, Asia-Pacific countries are also making fresh moves in response to Omicron.

Australia on Friday said it would commit an additional 540 million Australian dollars, or the equivalent of about $382 million, to Covid-19 response measures, citing concerns about the spread of the new variant. Some of the funds will be directed to research for new Covid-19 treatments, to better understand the immune response of children and vulnerable populations and to improve modeling on airborne transmission, among other things.

The country has 14 confirmed Omicron cases—including a student in Sydney who hadn’t traveled recently. Since Wednesday, New South Wales state, which includes Sydney and which is where most of the country’s Omicron cases have been recorded, increased penalties fivefold for those who skirt mandates for testing, isolation or quarantine.

A staff member walked through Hong Kong International Airport last month.

Photo: Chan Long Hei/Bloomberg News

The emergence of the Omicron variant highlights how the country needs “robust health measures to continue to underpin our Covid-19 health response,” Health Minister Greg Hunt said in a statement.

In Singapore, the city-state’s two confirmed Omicron cases were travelers arriving from southern African countries. The two individuals, who were fully vaccinated and had only mild symptoms, were isolated after arriving on Dec. 1 and didn’t interact with the local population, the health ministry said.

For other potential Omicron cases, close contacts of those who are infected will be quarantined at medical facilities for 10 days. That is an increase from the usual seven-day monitoring required for close contacts of other Covid-19 cases.

In South Korea, the threat of Omicron triggered a pullback in some softer restrictions that were enacted last month. Starting Monday, private gatherings in the Seoul metropolitan area will be limited to six people—down from 10—and everywhere else in the country they will be capped at eight people, a reduction from 12.

If a traveler is found to be infected with Covid-19 when arriving in South Korea, they will be subject to genomic surveillance testing to determine whether they have been infected by the Omicron variant. Family members and close contacts of an Omicron patient will be tracked down and tested for the virus within 24 hours of contact, accelerating the previous contact-tracing timeline.

—Jon Emont in Singapore and Natasha Khan in Hong Kong contributed to this article.

Write to Miho Inada at miho.inada@wsj.com, Dasl Yoon at dasl.yoon@wsj.com and Rhiannon Hoyle at rhiannon.hoyle@wsj.com