LONDON — As the still-mysterious Omicron variant reached American shores, the World Health Organization on Wednesday scolded wealthy countries that imposed travel bans and dismissed those that poured resources into vaccine booster campaigns when billions in poor countries had yet to receive their first shots.

The comments by W.H.O. officials reopened fraught questions of equity in how the world has handled the coronavirus pandemic since a stark divide over the availability of vaccines emerged between rich and poor countries earlier this year.

But amid fears of a new wave of Covid-19, that seemed unlikely to sway leaders in Europe, Asia, and the United States, which reported its first confirmed Omicron case, in California, on Wednesday. They are scrambling to shield their populations from the variant — about which much remains unknown — by topping up their protection and tightening restrictions on incoming travel.

Travelers reacted with confusion and dismay to news that the United States plans to toughen testing requirements and the screening of inbound passengers. That decision came after Japan, Israel, and Morocco barred foreign travelers and Australia delayed reopening its borders for two weeks.

“It’s a shame because travel just opened up again,” said Giritharan Sripathy, a Singaporean film producer, who was scheduled to fly to New York from London on Thursday. Mr. Sripathy, who had already taken a P.C.R. test three days before his flight, as required, said he had scheduled a new rapid test as a precaution, “in case they change the rules tomorrow.”

The W.H.O. added its voice to the objections, calling travel restrictions against southern African countries ineffective and unfair.

“Blanket travel bans will not prevent the spread of Omicron, and they place a heavy burden on lives and livelihoods,” Tedros Adhanom Ghebreyesus, the director general of the W.H.O., said at a news conference in Geneva.

He praised Botswana and South Africa, where the earliest cases of Omicron were detected last month, for promptly reporting the findings to international health authorities. “It is deeply concerning to me that these countries are now being penalized by others for doing the right thing,” he said.

Other W.H.O. officials said the bans on southern African countries by the United States and European countries would impede efforts to combat the variant since biological samples could not be transported out of those countries, even though they were willing to share them. The Omicron variant has been detected in two dozen countries by Wednesday — just six days after its existence was first revealed to the world — and Dr. Tedros warned that the number would rise.

The W.H.O. also voiced skepticism about ambitious booster plans that it claimed come at the expense of first-time vaccinations in less wealthy nations. Britain this week announced a massive new campaign to deliver booster shots to all adults by the end of January. Other European countries and the Biden administration are also pushing these shots as a first line of defense against the variant, buying time for scientists to unravel its genomic code.

W.H.O. officials, however, said there was not yet evidence that boosters prevented illness and hospitalization in people infected with variants. They suggested that the reserve of vaccines stockpiled by Britain and other countries could be better used in places where a large portion of the population was not yet vaccinated.

“It’s a luxurious position to be in, if you have enough vaccine to do that,” said Michael Ryan, the executive director of the W.H.O.’s health emergencies program, referring to the British campaign.

The renewed spotlight on inequities came as the W.H.O.’s member states took the first step toward what many governments hope will be a legally binding treaty to reinforce global defenses against pandemics.

The W.H.O. will set up a negotiating body to begin hammering out an international agreement intended to ensure a more coherent and equitable response to future outbreaks. But the United States and other countries have pushed for a weaker mechanism that would not carry legal obligations for member states.

The patchwork of travel restrictions imposed after the emergence of Omicron highlighted yet again that countries are now determined to act swiftly and unilaterally to protect their populations, with little regard for how it could affect their neighbors or even whether the measures will accomplish their stated goal.

But public-health experts expressed skepticism that the restrictions would prove effective.

“Travel restrictions can delay importation and widespread seeding, but not stop it,” said Devi Sridhar, head of the global public health program at the University of Edinburgh. “The variant will be found almost everywhere, I’m sure, in the coming days.”

The major unanswered question, she said, was the rate of transmission and whether Omicron could outcompete the Delta variant to establish itself as the dominant strain of the virus.

Professor Sridhar and other experts have called for greater global coordination, and the W.H.O.’s push for an international treaty is a step in that direction. Still, the United States, with support from Brazil and other countries, refused to commit to anything that is legally binding and kept open the possibility of a weaker agreement.

Proponents want commitments to share data, virus samples and technology, and to ensure equitable distribution of vaccines. But that raises politically sensitive questions of national sovereignty over access to the sites of outbreaks, and investigations into origins of diseases. China, for example, has resisted calls for an independent inquiry into the emergence of the coronavirus in the city of Wuhan in late 2019.

The Secretary General of the United Nations, António Guterres, who has described vaccine hoarding by wealthy nations as “an obscenity,” echoed the criticism of the international response to the Omicron scare.

“The people of Africa cannot be blamed for the immorally low level of vaccinations available to them,” he told reporters after meeting the chairman of the African Union Commission, Moussa Faki Mahamat of Chad. “Nor should they be collectively punished for identifying and sharing crucial science and health information with the world.”

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.