Symptoms also differ from those in countries where monkeypox is endemic.
"Our data reveals additional clues of ongoing viral evolution and potential human adaptation."
WHO has called for more case surveillance.
A Korean citizen tested positive after returning from Germany.
"It's definitely a sign that it's moved up the list in terms of important threats."
There have been 1,600 confirmed and 1,500 suspected cases of monkeypox this year and 72 deaths.
Hundreds of cases have now been reported outside parts of Africa, where the disease is endemic.
New cases in in man who returned from Europe.
More than 300 suspected and confirmed cases of monkeypox have been reported in May, mostly in Europe.