WASHINGTON - U.S. President Joe Biden on Thursday imposed stricter coronavirus testing requirements for international travelers entering the country. He also laid out a plan to fight the advance of the coronavirus pandemic in the United States over the coming winter months.

Each international traveler will be required to have a negative COVID-19 test within one day of departure for the United States, a change from the current three-day policy. The rule will apply to both U.S. citizens and foreign nationals.

In addition, Biden extended the mandate for wearing masks on public transportation and in airports from January 18 to March 18.

A second case of the omicron variant of the coronavirus was confirmed in the U.S. on Thursday. But Biden told a gathering of officials at the National Institutes of Health outside Washington that his plans would offer the country "a moment [where] we can put this crisis behind us."

Governments Announce Stricter COVID Rules, Prompting Worries About Overreaction

The Democratic president, locked in courtroom battles with Republican opponents over his demand that 84 million workers at companies with 100 employees or more be vaccinated by January 4, decried what he said was a "sad, sad commentary" that anti-coronavirus measures have turned into fractious political fights.

With the omicron variant spreading around the globe, Biden said, "we're going to fight this variant with science and speed, not chaos and confusion."

He outlined new efforts to get shots into the arms of 60 million unvaccinated Americans and urged 100 million people already inoculated to get booster shots.

The president said there now were 80,000 locations in the U.S. where people can get vaccinated and 35,000 sites where parents can get their children ages 5 to 18 vaccinated.

San Francisco health director Dr. Grant Colfax talks about the first confirmed case of the omicron variant as Mayor London Breed, right of podium, listens during a COVID-19 briefing outside City Hall in San Francisco, California, Dec. 1, 2021.

Biden said hundreds of new sites were being opened where families can go together to get everyone vaccinated at the same time. He said he would do "everything in my power" to advance research and approval of shots for children younger than 5 years old.

He said the government was making at-home COVID-19 tests more readily available with insurance companies paying the cost. Those without insurance will also be able to get the tests for free.

Biden also said 60 medical teams were being created to fly on short notice to states where hospitals have been overwhelmed with COVID-19 cases and need more health care workers.

He said the U.S. would ship 200 million more vaccine doses around the world in the coming weeks, but "not at the expense of any American" getting a shot first.

Dr. Anthony Fauci, the country's top infectious-disease expert and Biden's chief medical adviser, on Wednesday stressed the need for people to get vaccinated, including booster shots.

He said there was every reason to believe the rise in immune response provided by booster shots would help prevent severe disease if someone were infected by the omicron variant.

"I think what's happening now is another example of why it's important for people to get vaccinated who've not been vaccinated," Fauci said.

The second U.S. omicron case involved a Minnesota man who had attended a convention in New York City.

Medical workers check a COVID-19 patient at the intensive care unit (ICU) of Bagae Hospital in Pyeongtaek, South Korea, Nov. 29, 2021

The first confirmed U.S. case of someone infected with the omicron variant of the coronavirus was discovered earlier in the western state of California, U.S. health officials said. That person returned to the U.S. from a trip to South Africa on November 22 and tested positive on Monday, Fauci told reporters.

Fauci said the person had mild coronavirus symptoms, was self-quarantining and was improving. The person was fully vaccinated, he said, but had yet to get a booster shot.

The omicron case adds the U.S. to the growing list of at least 24 countries where the variant has been discovered.

The U.S. joins a growing list of nations that have imposed some form of travel restrictions or outright bans on foreign travelers since the omicron variant was first identified November 24 by scientists in South Africa, according to the World Health Organization.