The World Health Organization has declared that monkeypox is a public-health emergency of international concern, despite divisions among members of the committee of experts who advise the agency, as global case numbers surpass 16,000.

This is the first time the WHO has declared a global health emergency since the start of the Covid-19 pandemic in January 2020. In an unusual move, WHO Director-General Tedros Adhanom Ghebreyesus, went against the majority view of the emergency committee in making the declaration.

Monkeypox—rarely detected outside Africa before now—has in recent weeks spread to thousands of people across dozens of countries, mainly among men who have sex with men. No deaths have been reported among the cases outside of Africa, but three people have died in Nigeria and two in the Central African Republic since the start of the year. Epidemiologists say the virus, which requires close contact to spread, is likely exploiting close-knit social and sexual networks among men who have sex with men.

Dr. Tedros said nine members of the advisory panel were against declaring a public-health emergency of international concern, or PHEIC, and six were in favor. The committee didn’t hold a formal vote on its recommendation, but members expressed their views. The decision to declare a PHEIC is ultimately in the hands of the director-general, who considers the views of the committee, among other factors, to come to a decision.

SHARE YOUR THOUGHTS

What questions or concerns do you have about monkeypox? Join the conversation below.

That division reflects the challenging nuances of the monkeypox outbreak. Some emergency committee members expressed concerns that declaring a PHEIC might cause undue alarm among the general public when the disease is currently affecting a specific group of people, according to a report of the meeting. They also were concerned that declaring a PHEIC could encourage stigmatization of that group, hampering efforts to control the virus.

But those in favor of declaring a PHEIC believed that doing so would mobilize more resources to tackle the outbreak. They also voiced concerns about the occurrence of cases in pregnant women and children that, while small, were reminiscent of the early days of the HIV pandemic. Those members also pointed to gaps in knowledge about monkeypox, such as possible modes of transmission and the effectiveness of the vaccines and treatments used to fight the disease. Declaring a PHEIC could release more research funding toward efforts to answer these questions, they said.

The move is effectively a call to arms for governments to step up their responses to an emerging health threat. In practice, though, the WHO doesn’t have the power to compel governments to do so. The WHO’s director-general can also make recommendations to the international community, although they aren’t legally binding.

It is unclear whether a PHEIC declaration has the desired effect, said Clare Wenham, associate professor of global health policy at the London School of Economics. “The assumption that it’ll suddenly open up financing is an assumption,” she said. “We simply don’t know what a PHEIC does.”

Many governments, including the U.S., have taken steps to curb the spread of monkeypox, for example by offering vaccination to those most at risk of exposure to the virus, although some local public-health leaders say they need more funding to mount an effective response. The existing action by governments was among the reasons given by some committee members that a PHEIC wasn’t necessary.

The WHO issued recommendations for different groups of countries. For countries experiencing an outbreak, the recommendations included engaging and protecting the affected communities, intensifying surveillance and public-health measures, and accelerating research into the use of vaccines, treatments and other tools.

The WHO had previously convened its emergency committee in June but said the outbreak didn’t at that point constitute a global health emergency. Back then, the disease had reported 3,040 cases across 47 countries.

Unusual for an emerging disease, there are already vaccines and treatments that can be used to counter monkeypox. That is because some governments have invested in developing defenses against the accidental or deliberate reintroduction of smallpox, a closely related but much more severe virus. Some countries hold these treatments and vaccines in national stockpiles, but they aren’t readily available everywhere.

In some places, including the U.S., U.K. and parts of Canada, broad groups of men who have sex with men are being offered vaccination in an effort to slow the spread, although vaccine supplies have so far been constrained. Public-health authorities also are working to raise awareness among men who have sex with men about the spread of monkeypox.

People waited to get the monkeypox vaccine in Brooklyn, N.Y., earlier this month.

Photo: kena betancur/Agence France-Presse/Getty Images

The U.S. has so far reported more than 2,800 cases from various parts of the country. A group of about 50 House Democrats this week called on the Biden administration to declare monkeypox a public-health emergency. The Department of Health and Human Services said Friday it was considering whether to take this action.

Governments are mainly using a smallpox shot made by Danish vaccine maker Bavarian Nordic A/S that was developed as a safer alternative to older smallpox vaccines. That vaccine, known as Jynneos in the U.S., was primarily intended for use as a smallpox defense, but the Food and Drug Administration also authorized it for monkeypox when it approved the shot in 2019. Another, older smallpox vaccine, called ACAM2000, also is available in the U.S. but is rarely used because it has potentially serious side effects. A third vaccine, called LC16, is currently available only in Japan, according to Tim Nguyen, WHO’s head of high-impact events preparedness.

WHO officials said Saturday that despite the disease’s growing spread, they believed there was still an opportunity to bring it under control with the tools available.

“We don’t know for sure if we’ll be able to support countries enough, and communities enough, to stop this outbreak,” said Rosamund Lewis, the WHO’s technical lead for monkeypox. “We think it is still possible precisely because it remains primarily in one group who are very active in health-seeking behavior and supporting each other in reducing risk.”

The sudden global spread of monkeypox follows increasing concerns in parts of Africa about its growing presence there. For decades, the disease was detected mainly in the forests of the Congo Basin in central Africa among people who caught it from wild animals they had hunted, handled or eaten. But five years ago, Nigeria experienced an outbreak in some urban and suburban areas after nearly 40 years without a case. Although Nigerian public-health officials contained that flare-up, a few dozen cases have been recorded in the country most years since.

Researchers in the Democratic Republic of Congo, which has for decades grappled with monkeypox, have said the virus is now spreading differently there. Monkeypox is showing up in different parts of the country, not just in the rainforest regions where it was historically seen.

The WHO is also urging researchers to investigate poorly understood aspects of monkeypox such as whether it is possible to have the disease without showing symptoms or whether it can be spread in ways other than close contact.

The symptoms of monkeypox also appear to be more varied than in earlier outbreaks in Africa. Classic monkeypox starts with flulike symptoms including fever and aches, with the later appearance of a rash usually starting on the face. In the current outbreak, doctors have described some cases in which the rash appears before a fever and other cases in which the rash remains concentrated in the genital area, for example.

The WHO had previously applied the designation of a public-health emergency of international concern on six occasions: H1N1 swine flu in 2009, polio in 2014, Ebola in 2014, Zika in 2016, Ebola again in another outbreak in 2019 and Covid-19 in 2020.

Write to Denise Roland at Denise.Roland@wsj.com