File photo of WHO chief scientist Soumya Swaminathan (AFP)
Fighting the virus required "intense" efforts, said the WHO, calling for broad data collection and sharing on how well vaccines work against it.
Experts have detected a surge of monkeypox cases since early May outside of the West and Central African countries where the disease has long been endemic. Most of the new cases have been in western Europe.
"I am concerned about sustained transmission because it would suggest that the virus is establishing itself and it could move into high-risk groups including children, the immunocompromised and pregnant women," WHO chief Tedros Adhanom Ghebreyesus told reporters.
"We are starting to see this, with several children already infected."
There are two cases aged under 18 in Britain.
WHO chief scientist Soumya Swaminathan called for "very careful studies of the vaccine in different population groups ... so that we get data that's broadly applicable, and also to ensure that children, pregnant women and the immunosuppressed are considered for inclusion in these trials".
As of June 22 this year, 3,413 laboratory-confirmed cases and one death have been reported to the WHO, from 50 countries.
WHO emergencies director Michael Ryan said that countries with stockpiles of vaccines, led by the United States, had shown willingness to share them.
However, they had largely only been licensed for use against monkeypox's far more severe relative smallpox, which is also caused by an orthopoxvirus and which is the only disease successfully eradicated through vaccination.
"It's really important as we encourage the sharing of these products that we also collect the necessary clinical efficacy data," said Ryan.