The World Health Organization said on Sunday that monkey pox constitutes a "moderate risk" to overall public health at global level after cases were reported in countries where the disease is not typically found.

"The public health risk could become high if this virus exploits the opportunity to establish itself as a human pathogen and spreads to groups at higher risk of severe disease such as young children and immunosuppressed persons," WHO said.

As of May 26, a total of 257 confirmed cases and 120 suspected cases have been reported from 23 member states that are not endemic for the virus, the health agency said in a statement. There has been no reported fatalities so far.

WHO also said that the sudden appearance of monkeypox at once in several non-endemic countries suggests undetected transmission for some time and recent amplifying events.

The agency added that it expects more cases to be reported as surveillance in endemic and non-endemic countries expands.

Monkeypox is an infectious disease that is usually mild, and is endemic in parts of west and central Africa. It is spread by close contact, so it can be relatively easily contained through measures such as self-isolation and hygiene. 

Most of the cases reported so far have been detected in the UK, Spain and Portugal.

"The vast majority of reported cases so far have no established travel links to an endemic area and have presented through primary care or sexual health services," the U.N. agency said.