The number of registered monkeypox cases in countries not previously affected by the virus has climbed to more than 1,000 since May.
WHO chief, Tedros Adhanom Ghebreyesus said this on Wednesday in Geneva.
The number of infections recorded in Germany rose to 113 on Wednesday, a sharp rise from the previous day when the toll stood at 80.
The figures come from the national disease control, the Robert Koch Institute, which said that all those affected so far were men.
The UK Health Security Agency announced that additional cases of monkeypox had been detected in England and Scotland, bringing the combined British total to 321 as of Tuesday.
According to the WHO, it is not clear whether an infection, unlike smallpox, which was eradicated in 1980 gives lifelong immunity.
Cases are known from Africa in which people have been re-infected after recovery, monkeypox expert Rosamund Lewis said.
While individual infections have been reported in women, so far the majority concern men who have sex with men, she said.
It is important for those affected to isolate themselves if they show symptoms and to avoid contact with family members, Lewis stressed.
The virus is usually transmitted through physical contact.
WHO is currently assessing how much smallpox vaccine is available worldwide.
According to the body, countries have stocks to guard against a possible new outbreak, because it comes from the same family of viruses, this vaccine is also effective against monkeypox.
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