President Biden will restrict travel from South Africa and seven other African countries to try to contain a troubling new variant of the coronavirus, senior administration officials said on Friday, though they said it would be impossible to prevent it from entering the United States.

Starting on Monday, the administration will prohibit travelers from South Africa, Botswana, Zimbabwe, Namibia, Lesotho, Eswatini, Mozambique and Malawi from coming to the United States, the officials said.

The travel ban will not apply to American citizens or lawful permanent residents, officials said. But they will need to show a negative coronavirus test before coming to the United States.

Mr. Biden made the decision after he was briefed by advisers including Dr. Anthony S. Fauci, who said in an interview Friday that the variant appeared to be spreading rapidly and that he and other health officials in the United States were consulting with South African scientists.

The White House announced the decision after the World Health Organization said the newly detected version of the virus, labeled Omicron, was “a variant of concern,” a category for dangerous variants that may spread quickly, cause severe disease or decrease the effectiveness of vaccines or treatments.

“I’ve decided that we’re going to be cautious,” Mr. Biden told reporters in Nantucket. “We don’t know a lot about the variant except that it is a great concern and seems to spread rapidly.”

By imposing the travel restrictions, the administration will not stop the virus from coming to the United States; in fact, experts said, it may already be here. But it can give health officials and pharmaceutical companies time to determine whether the current vaccines work against the new variant — and if not, to create new vaccines that do.

“It’s going to buy us some time,” Dr. Fauci said. “It’s not going to be possible to keep this infection out of the country. The question is: Can you slow it down?”

Dr. Fauci said the new variant has about 30 mutations, and roughly 10 of them are on a part of the virus that is associated with transmissibility and immune protection. That suggests the virus may be more transmissible and may escape the current vaccines “to an extent yet to be determined.”

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.