The Biden administration is tightening travel rules to and within the U.S., requiring all in-bound international passengers to test for Covid within 24 hours of departure and extending its mask requirement on all domestic flights and public transportation through March 18.

The changes were announced Thursday as part of a broader plan to bolster the nation’s arsenal of tools in its fight against the virus as the world enters its third year of the pandemic. 

The plan is also the latest move by the Biden administration to stem the spread of the new, highly mutated omicron Covid-19 variant, which was first reported to WHO by South Africa a week ago. At least 23 countries have identified omicron cases so far, and that number is expected to rise in the coming days and weeks, the WHO said Wednesday. 

The U.S. joined that list after confirming its first case of the variant in Northern California on Wednesday. 

The tightened pre-departure testing protocols will apply to all in-bound international travelers regardless of vaccination status and will begin as early as next week, senior administration officials told reporters during a press call late Wednesday. Previously, the U.S. required proof of a negative Covid test taken within 72 hours of departure, though this was tightened to only vaccinated travelers last month. Unvaccinated travelers had to have a negative Covid test within one day of departure.

“Our doctors believe tightening testing requirements for pre-departure will help catch more cases, potential cases of people who may be positive and inside the country,” a senior administration official said. “And so now is the right time to do it. And we can implement it very quickly.”

The mask mandate, which was originally supposed to expire in January, requires masking while on buses, trains, planes and transportation hubs such as indoor bus terminals and airports. Fines will remain doubled from their initial levels for noncompliance with the requirement, starting at $500, and going up to $3,000 for repeat offenders.

The Biden administration’s new plan also urges businesses to “move forward expeditiously” with requiring workers to get vaccinated or tested weekly, according to the plan. Such protocols are “especially important” given the new omicron variant, the plan added. 

“We’re asking businesses to step forward and do what’s right to protect our workers, our communities — to put in place some sort of vaccination requirement or testing requirements for the workplace,” a senior administration official said during the call. 

The administration had given businesses with 100 or more employees until Jan. 4 to ensure their staff are either vaccinated against Covid-19, or submit a negative test weekly before entering the workplace. Unvaccinated employees were also supposed to start wearing masks indoors at the workplace on Dec. 5.

However, the Occupational Safety and Health administration suspended enforcement and implementation of the requirements earlier this month, after the U.S. Court of Appeal for the Fifth Circuit halted the policy pending review. 

More than two dozen lawsuits have been filed to overturn the Biden policy. But labor unions are asking the courts to expand the requirements to cover smaller businesses and protect more workers.

The Biden administration’s winter Covid plan also includes more free at-home tests.

More than 150 million Americans with private insurance will be able to get reimbursed for their at-home tests, according to the plan. Those without private insurance will be able to access at-home tests through community sites such as health centers and rural clinics.

The plan also aims to bolster outreach efforts to get more people vaccinated, ensure the “quitable distribution of Covid -19 treatments to vulnerable populations and launch over 60 rapid response teams to states to help combat the virus.

“While this new variant is a cause for concern, it is not a cause for panic,” a senior administration said. “We have the tools we need to confront this variant and keep making progress in our fight against the virus.”

Last week, Biden ordered precautionary air travel restrictions from South Africa, Botswana, Zimbabwe, Namibia, Lesotho, Eswatini, Mozambique and Malawi. He did not indicate how long the bans, which took effect Monday, will remain in place. 

Those restrictions came into effect three weeks after the U.S. lifted a blanket ban on visitors from more than 30 countries, including the U.K., India, Brazil and South Africa, replacing them with vaccination requirements for tourists. Other countries implemented new travel restrictions last week, sending airline and other travel stocks tumbling.

The president on Monday also said his administration is working with vaccine manufacturers Pfizer, Moderna and Johnson & Johnson to develop contingency plans for additional vaccines or boosters tailored to protect against the new variant. 

He is directing the U.S. Food and Drug Administration and the CDC to use the “fastest process available without cutting any corners” to approve such potential vaccines. 

Current Covid-19 vaccines on the market are believed to provide some protection against omicron, Biden said Monday, adding that booster shots “strengthen that protection significantly.” 

On Wednesday, Fauci noted the profile of the variant suggests its mutations could reduce the effectiveness of current vaccines. However, more data is still needed, he added. 

Omicron has more than 30 mutations to the spike protein alone. Some of the mutations are associated with higher transmission and a decrease in antibody protection, according to the WHO. 

CNBC’s Spencer Kimball and Leslie Josephs contributed to this report.