"Investigations are ongoing, but the sudden appearance of monkeypox in many countries at the same time suggests there may have been undetected transmission for some time," World Health Organization chief Tedros Adhanom Ghebreyesus told reporters.
Since Britain first reported a confirmed monkeypox case on May 7, more than 550 confirmed cases of the disease have been verified in 30 countries outside of the west and central African nations where it is endemic, the WHO said.
The UN health agency's top monkeypox expert Rosamund Lewis said that the appearance of so many cases across much of Europe and other countries where it has not been seen before "is clearly a cause for concern, and it does suggest undetected transmission for a while".
"We don't know if it is weeks, months or possibly a couple for years," she said, adding that "we don't really know if it is too late to contain".
Monkeypox is related to smallpox, which killed millions around the world every year before it was eradicated in 1980.
But monkeypox, which spreads through close contact, is much less severe, with symptoms typically including a high fever and a blistery chickenpox-like rash that clears up after a few weeks.
So far, most cases have been reported among men who have sex with men, although experts stress there is no evidence that monkeypox is transmitted sexually.
"Anyone can be infected with monkeypox if they have close physical contact with someone else who is infected," Tedros said.
He urged everyone to help "fight stigma, which is not just wrong, it could also prevent infected individuals from seeking care, making it harder to stop transmission".
The WHO, he said, was also "urging affected countries to widen their surveillance".
Lewis insisted it was vital "that we collectively all work together to prevent onward spread", through contact tracing and isolation of people with the disease.
Vaccines developed for smallpox have also been found to be about 85 percent effective in preventing monkeypox, but they are in short supply.
WHO is not proposing mass-vaccination, but rather targeted use in some settings to protect health workers and people most at risk of infection.
Lewis highlighted that monkeypox cases had also been on the rise in endemic countries, where thousands fall ill from the disease each year, with around 70 deaths from the virus reported across five African countries so far this year.
The fatality rate for monkeypox is usually quite low, and no deaths have been reported among the cases found so far outside of endemic countries.
But Maria Van Kerkhove, the WHO lead on emerging diseases, warned that while no deaths had been reported, that could change if the virus got into more vulnerable populations.