President Biden added travelers from South Africa to the list of those barred from entering the United States — the latest step in an effort to help contain the spread of the coronavirus and fast-moving variants that have surfaced in that country and others in recent weeks.

Biden also extended a ban on travelers from Brazil, the United Kingdom, Ireland and 26 other European countries that had been set to expire Tuesday under a proclamation signed by President Donald Trump shortly before he left office.

“With the pandemic worsening and more contagious variants spreading, this isn’t the time to be lifting restrictions on international travel,” White House spokeswoman Jen Psaki said during a press briefing on Monday. She said the proclamation is part of the administration’s “science-driven response” to the coronavirus.

The restrictions on travelers from South Africa are set to take effect Saturday.

In an interview on “CBS This Morning,” Anthony S. Fauci, the government’s top infectious-diseases expert, said extending the ban and including South Africa “clearly will be helpful.” He said it is prudent to restrict travel of non-U.S. citizens.

“We have concern about the mutation that’s in South Africa,” Fauci said. “It is clearly different and more ominous than the one in the U.K.”

While some have questioned the value of travel bans, Fauci said one important difference this time around is that the ban dovetails with a requirement that all international travelers to the United States show proof of a negative coronavirus test.

This month, the Centers for Disease Control and Prevention warned that the variant of the coronavirus first seen in the United Kingdom would become the dominant strain in the United States within about two months. The South African variant, however, has not yet been identified in the United States.

On Monday, Moderna said its vaccine will protect people from both the U.K. and South African variants of the virus. However, the vaccine’s response to the South African variant was slightly diminished, officials said. As a result, the company said it would work to develop a new vaccine that can be added to the current two-dose regimen.

A study published this month, but not yet peer reviewed, found that the Pfizer-BioNTech vaccine also works against a key mutation in the U.K. and South African variants. More experiments and direct tests of the variants are expected to provide further information.

The Biden administration is also taking steps to require travelers to quarantine after their arrival in the United States and be tested a second time. Details on how those measures will be implemented are expected in the coming days.

Restrictions on travelers from 26 countries in Europe have been in place since mid-March, while Brazil was added to the list in May.

Under the ban, most non-U.S. citizens who have been in those countries in the past 14 days are barred from entering the United States. However, some exceptions are in place.

The extension of the ban comes as a new testing requirement for international travelers is set to take effect Tuesday.

Under the requirement announced earlier this month by the CDC, all international travelers to the United States will be required to show proof of a negative coronavirus test before boarding a flight. PCR and antigen tests will be accepted.

Those who refuse to be tested will not be allowed to fly. Travelers must be tested within three days before boarding their flights, and airlines will be required to review travelers’ documentation.

“As variants of the SARS-CoV-2 virus continue to emerge in countries around the world, there is growing evidence of increased transmissibility of some of these variants, as well as unknown health and vaccine implications,” the CDC said in a statement. “Testing before and after travel is a critical layer to slow the introduction and spread of COVID-19 and emerging variants.”

However, the agency said it would no longer issue waivers to countries with a limited ability to test travelers. The CDC had previously said it would allow carriers to apply for two-week waivers from the requirement.

“With the U.S. already in surge status, the testing requirement for all air passengers will help slow the spread of the virus as we work to vaccinate the American public,” the agency said.

In addition to requiring proof of a negative test, Biden also has directed officials to explore the feasibility of requiring international travelers to quarantine or self-isolate once they arrive in the United States.

The reinstatement of the ban is a blow to airlines that had hoped testing could replace bans and quarantines. Industry surveys have shown that people are more reluctant to travel if they must quarantine.

Since taking office, Biden has acted quickly to impose rules intended to reduce the spread of the virus as his administration seeks to ramp up efforts to vaccinate as many people as possible.

Last week, he signed an order mandating masks in airports and on many planes, trains, ships and intercity buses. The order followed one the president signed Wednesday, requiring that people wear masks on federal property.