Cases of monkeypox are being investigated in several European countries as well as the US, Canada and Australia, according to health authorities and local media reports.
The new cases were reported in Belgium, France, Australia and Germany.
This follows infections confirmed in Italy, Sweden, Spain, Portugal, the US, Canada and the UK - where the first European case was reported.
Monkeypox is most common in remote parts of Central and West Africa.
Instances of the disease outside of the region are often linked to travel to the area.
Monkeypox is a rare viral infection which is usually mild and from which most people recover in a few weeks, according to the UK's National Health Service.
The virus does not spread easily between people and the risk to the wider public is said to be very low.
The first case of the disease in the UK was reported on 7 May. The patient had recently travelled to Nigeria, where they are believed to have caught the virus before travelling to England, the UK Health Security Agency said.
There are now 20 confirmed cases in the UK, Health Secretary Sajid Javid said on Friday.
There is no specific vaccine for monkeypox, but a smallpox jab offers 85% protection since the two viruses are quite similar.
Authorities in the UK said they had bought stocks of the vaccine and started offering it to those with "higher levels of exposure" to monkeypox.
Spanish health authorities have also reportedly purchased thousands of smallpox jabs to deal with the outbreak, according to Spanish newspaper El País.
Australia's first case was detected in a man who fell ill after travelling to the UK, the Victorian Department of Health said.
In North America, health authorities in the US state of Massachusetts confirmed that a man has been infected after recently travelling to Canada. He was in "good condition" and "poses no risk to the public", officials said.