He said there had been some breakthrough infections among those who had recovered from the Delta variant, and among those who were vaccinated.

But at the same time, he said, scientists do not know the severity of the infections caused by the new variant. It is entirely possible that it spreads more quickly but causes less severe disease.

“You don’t want to say don’t worry, and you don’t want to say you’ve got to worry yourself sick, because we’re gathering information rapidly,” he said, adding, “Even though the numbers are still small, the doubling time is pretty rapid and the slope of the increase is really rather sharp.”

Biden administration officials said they were continuing to work with health officials in other countries to learn more about the variant.

“Restricting travel is going to slow its coming, not stop it from coming,” said Dr. Ezekiel Emanuel, chair of the department of medical ethics at the University of Pennsylvania and an adviser to the president during his transition. “The fact that it’s coming here is inevitable. The environment in which it comes may not be inevitable. We can alter the environment.”

Mr. Biden said on Friday that the rise of the Omicron variant was another reason for vaccinated Americans to get boosters and unvaccinated Americans to get inoculated — a point Dr. Fauci echoed. And Mr. Biden said the development should push the international community to donate more vaccines to nations suffering from a lack of access or poor vaccination rates.

Michael Osterholm, an infectious disease expert at the University of Minnesota who also advised Mr. Biden during his transition, said the administration had little choice on implementing the travel ban.

But Dr. Osterholm said it could take time before scientists know if the current vaccines are effective against the variant, and how transmissible it is. One way to figure that out is through laboratory studies, which will take several weeks, he said. Another way is to follow breakthrough cases in people who are already vaccinated, which could take months.