"I have decided that the global monkeypox outbreak represents a public health emergency of international concern," WHO chief Tedros Adhanom Ghebreyesus said at a press conference, according to AFP.
He said a committee of experts who met on Thursday was unable to reach a consensus, so it fell on him to decide whether to trigger the highest alert possible.
"WHO's assessment is that the risk of monkeypox is moderate globally and in all regions, except in the European region where we assess the risk as high," he added.
Monkeypox has affected over 15,800 people in 72 countries, according to a tally by the US Centers for Disease Control and Prevention (CDC) published on July 20.
A surge in monkeypox infections has been reported since early May outside the West and Central African countries where the disease has long been endemic.
On June 23, the WHO convened an emergency committee of experts to decide if monkeypox constitutes a so-called Public Health Emergency of International Concern (PHEIC) – the UN health agency's highest alert level.
But a majority advised Tedros that the situation, at that point, had not met the threshold.
The second meeting was called on Thursday with case numbers rising further, where Tedros said he was worried.
A viral infection resembling smallpox and first detected in humans in 1970, monkeypox is less dangerous and contagious than smallpox, which was eradicated in 1980.