The rapidly spreading monkeypox outbreak represents a global health emergency, the World Health Organisation's highest level of alert, WHO Director-General Tedros Adhanom Ghebreyesus said on Saturday.
The WHO label - a "public health emergency of international concern" - is designed to sound an alarm that a coordinated international response is needed and could unlock funding and global efforts to collaborate on sharing vaccines and treatments.
Members of an expert committee that met on Thursday to discuss the potential recommendation were split on the decision, two sources who spoke on the condition of anonymity told Reuters earlier, but the final decision falls to the UN agency's director-general.
Announcing his decision to declare the health emergency during a media briefing in Geneva, Tedros confirmed that the committee had failed to reach a consensus, with nine members against and six in favour of the declaration.
Previously, Tedros has typically endorsed expert committee recommendations, but the sources said he had likely decided to back the highest alert level due to concerns about escalating case rates and a short supply of vaccines and treatments, despite the lack of a majority opinion.
Lawrence Gostin, a professor at Georgetown Law in Washington, D.C who follows the WHO, said he applauded the political bravery of the agency.
"It does nothing but burnish the stature of WHO. The right result is clear – not declaring an emergency at this point would be a historic missed opportunity."
So far this year, there have been more than 16,000 cases of monkeypox in more than 75 countries, and five deaths in Africa.
The viral disease - which spreads via close contact and tends to cause flu-like symptoms and pus-filled skin lesions - has been spreading chiefly in men who have sex with men in the recent outbreak, outside Africa where it is endemic.
Until now, the label had only been applied to the coronavirus pandemic and ongoing efforts to eradicate polio.
The WHO and national governments have been facing intense pressure from scientists and public health experts to take more action on monkeypox.
Cases of the viral disease have ballooned since the committee first met at the end of June, when there were only about 3,000 cases.
At the time, the expert group agreed to reconsider their position on the emergency declaration if the outbreak escalated.
One of the key issues driving a reassesment was whether cases - that are almost entirely proliferating amongst men who have sex with men - would bleed into other groups, particularly children or others who have been vulnerable to the virus in past outbreaks in endemic countries.
On Friday, the United States identified its first two monkeypox cases in children.
Any changes to the virus itself could also spark a rethink, the committee had said.
The group is now split between those who think an emergency declaration would accelerate efforts to contain the disease, and those who do not think the above criteria have been met because the disease has not yet spread to new groups of people or had a high fatality rate, the sources said.