WASHINGTON —

       The Biden administration is expected to take steps in the coming days to toughen testing requirements for international travelers to the U.S., both vaccinated and unvaccinated, amid the spread of the new Omicron variant of the coronavirus.

       The precise testing protocols were still being finalized ahead of a speech by President Biden planned for Thursday on the nation’s plans to control the COVID-19 pandemic during the winter season, according to a senior administration official who said some details could still change. Among the policies being considered is a requirement that all air travelers to the U.S. be tested for COVID-19 within a day of boarding their flight. Currently, those who are fully vaccinated may present a test taken within three days of boarding.

       Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said Tuesday that the agency “is evaluating how to make international travel as safe as possible, including predeparture testing closer to the time of flight and considerations around additional post-arrival testing and self-quarantines.”

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       The expected move comes after the U.S. largely reopened its borders to fully vaccinated foreign travelers on Nov. 8.

       Much remains unknown about the new variant, which has been identified in more than 20 countries but not in the U.S., including whether it is more contagious, whether it makes people more seriously ill and whether it can thwart COVID-19 vaccines. Dr. Anthony Fauci, the top U.S. infectious-disease expert, said more will be known about Omicron in two to four weeks as scientists grow and test lab samples of the variant.

       On Monday, as he sought to quell public concern about Omicron, Biden said that in his Thursday remarks, “I’ll be putting forward a detailed strategy outlining how we’re going to fight COVID this winter — not with shutdowns or lockdowns but with more widespread vaccinations, boosters, testing and more.”