WASHINGTON—President Biden plans to tighten up Covid-19 testing timelines for travelers entering the U.S. and extend a mask mandate on airplanes and other public transportation as part of a broad administration effort to combat the Omicron variant.

International travelers coming to the U.S. will have to test within a day of departure, regardless of vaccination status, rather than the 72 hours currently required for vaccinated travelers, under new protocols early next week, senior administration officials said. The new testing...

WASHINGTON—President Biden plans to tighten up Covid-19 testing timelines for travelers entering the U.S. and extend a mask mandate on airplanes and other public transportation as part of a broad administration effort to combat the Omicron variant.

International travelers coming to the U.S. will have to test within a day of departure, regardless of vaccination status, rather than the 72 hours currently required for vaccinated travelers, under new protocols early next week, senior administration officials said. The new testing rules will apply both to U.S. citizens and foreign nationals entering the country.

The administration will also require travelers to wear masks through mid-March on planes, buses and trains, and at domestic transportation hubs such as airports and indoor bus terminals, rather than allowing the requirement to expire on Jan. 18 as planned. Fines will continue to be double their initial levels, with a minimum fine of $500 for noncompliance and up to $3,000 for repeat offenses.

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

The new steps came as the U.S. reported its first cases of the Omicron variant. Top health officials said Wednesday that a fully vaccinated traveler who recently returned to California from South Africa has mild Covid-19 symptoms that are improving and that all known contacts had tested negative for the variant. On Thursday, health officials confirmed another case, a Minnesota resident who had recently traveled to New York City for a convention, experienced mild symptoms and has since recovered.

Mr. Biden is trying to curb the virus’s spread and encourage precautions without endorsing the lockdowns and mask mandates that were widespread in earlier stages in the pandemic. Administration officials say that the pandemic is in a new phase, with vaccines and boosters in place, and that they can mitigate the spread of new variants without the economic and social disruptions that marked 2020.

President Biden is expected to announce a public-education campaign that will target seniors.

Photo: Oliver Contreras/Zuma Press

Mr. Biden centered his 2020 campaign on steering the nation out of the pandemic, and his approval rating in polls took a hit following the spread of the Delta variant and the chaotic troop withdrawal from Afghanistan. Asked Monday if he thought local and state officials should reinstate mask requirements that have been lifted, he said he encouraged all Americans to wear them when indoors in crowded settings but didn’t mention mandates.

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“If people are vaccinated and wear their masks, there’s no need for the lockdowns,” he said in response to another question.

Republicans have seized on the news surrounding the Omicron variant to criticize Mr. Biden’s handling of the pandemic. Some are seeking to include a measure in coming government-funding legislation aimed at blocking the Biden administration from enacting rules requiring employers with 100 or more employees to mandate vaccines or test workers weekly.

The new variant has a constellation of mutations that could make it spread more easily and evade some vaccine protection. Newly reported cases of Covid-19 almost doubled Wednesday from Tuesday in South Africa, the country with the highest number of identified cases of the Omicron variant. The confirmed U.S. cases raise the number of countries where Omicron has been identified to at least 24 on five continents since scientists in South Africa first identified it in late November.

Governments around the globe, including the U.S., have restricted travel from southern Africa in response to the Omicron variant’s emergence, though officials are divided over how well travel bans stop a new variant from moving around.

Mr. Biden’s top Covid-19 advisers have sought to emphasize that existing vaccines and boosters should provide sufficient protection against the variant.

Both PCR and antigen tests will be accepted for international travel; a person being tested for Covid-19 at Los Angeles International Airport.

Photo: Mario Tama/Getty Images

Other administration actions include discussions with South African scientists to learn more about the severity and transmission rate of the variant, as well as outreach to vaccine makers and academic labs involved in any possible development of new tests, vaccines and treatments, senior administration officials said.

The administration has also expanded surveillance at four of the busiest U.S. airports: New York’s John F. Kennedy airport; Newark, N.J.; San Francisco and Atlanta.

Information on the severity of Omicron won’t be available for about two weeks, so the U.S. and other countries are rushing to prepare for worst-case scenarios should the variant prove far more transmissible or severe.

The World Health Organization on Nov. 26 identified Omicron as a variant of concern.

The administration’s plan includes reimbursing health providers in Medicaid who talk to families about getting children vaccinated, as well as new guidance clarifying that over-the-counter Covid-19 tests are covered 100% by health insurers. The administration will also provide more free tests to community health sites.

Details about insurance coverage for at-home tests will be spelled out in guidance to be released by Jan. 15 from the departments of Health and Human Services, Labor and Treasury. The guidance will stipulate that people who buy the tests will be able to seek reimbursement from their group health plan or health insurer and have it covered during the public-health emergency.

The administration is ramping up its push around booster shots to combat the Omicron variant; a vaccine site in Sanford, Fla.

Photo: Paul Hennessy/Zuma Press

The administration has authority to do that under legislation that Congress passed in March that required group health plans and issuers to cover diagnostic Covid-19 testing, senior administration officials said. Rapid tests have sometimes been in short supply, but more have been authorized in recent months.

Both PCR and antigen tests will be accepted for international travel. The administration has been working to make tests more readily available. Mr. Biden in the fall said the U.S. aims to quadruple the supply of at-home tests by the end of the year by investing a total of $3 billion in the production and purchase of rapid tests.

The administration is also ramping up its push around booster shots, and Mr. Biden on Thursday is expected to announce a public-education campaign that will target seniors and a program with pharmacies to reach out to Americans who are eligible for a booster.

Officials said the U.S. will also expedite the delivery of vaccines overseas, with plans to ship 200 million doses to countries in need in the next 100 days. The Biden administration has pledged to share a total of 1.2 billion doses globally, though in the wake of the new variant some public-health advocates have renewed criticism of the U.S. and other wealthy nations for administering boosters when developing countries still lack vaccine access.

The administration recently initiated new international travel protocols, including a requirement for most foreign travelers to be vaccinated, as it relaxed a series of travel bans on many countries. The administration imposed a temporary ban on non-U.S. citizen travelers from South Africa and seven other southern African nations following the emergence of the Omicron variant.

Write to Stephanie Armour at stephanie.armour@wsj.com and Sabrina Siddiqui at Sabrina.Siddiqui@wsj.com