The borderless nature of the virus, Mr. Guterres said, means that “travel restrictions that isolate any one country or region are not only deeply unfair and punitive — they are ineffective.”

Although the United States is not weighing the kind of blanket travel ban on foreign visitors imposed by Japan, the restrictions being weighed by the Centers for Disease Control and Prevention in the United States are stirring widespread concern. The agency is considering requiring travelers to provide a negative result from a test taken within 24 hours before departure, a spokesman said on Tuesday night.

Though the C.D.C. has yet to officially announce the changes, the prospect sent travelers searching for updates, booking pre-emptive tests where they could, and scouring airline websites for reservation changes, as the pandemic threatened to upend another December travel season.

Carlos Valencia, a dual Spanish-American citizen whose Seville-based company operates a study abroad program for American students, had planned to return to the United States in January. But he said that he would put the trip on hold until “there is at least some clarity about whether the new rules make a trip feasible.”

Whatever shape the restrictions take, he said, they are “way overdone — especially when you consider how lax the U.S.A. has been with getting people to wear face masks and its own health safety measures.”

Emanuela Giorgetti, a teacher in northern Italy, was hoping to join her fiancé, whom she has not seen for almost two years, for Christmas in Chicago. “When I heard the news,” she said, “I thought, ‘Here we go again.’”

Given the potential threat posed by Omicron, she said she understood the impulse to tighten the rules. But it still seemed unfair.

“We have more vaccinated people in Italy than in the U.S., we wear masks indoors and try to go by the rules,” Ms. Giorgetti said.

Reporting was contributed by Nick Cumming-Bruce, Rick Gladstone, Raphael Minder, Gaia Pianigiani, Michael D. Shear and John Yoon.

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