UK health officials say they cannot tell if the spread of monkeypox has peaked in the country as they announced another 45 cases, bringing the total in the disease's biggest-ever outbreak beyond Africa to 366 cases.

The United Kingdom Health Security Agency said 99 per cent of the total cases were in men and that nearly all of the 152 men who provided detailed information identified as gay, bisexual or men who have sex with men.

About 80 per cent of cases were in London and the median age of the people infected was 38, the agency said.

"We cannot yet determine if transmission has stopped increasing," the agency said in a report, citing the reporting delay between when patients experienced symptoms and were confirmed as having monkeypox.

Anyone, regardless of their sexual orientation, is susceptible to the monkeypox virus if they come into close physical contact with an infected person or their bedsheets or clothes.

"Findings show that monkeypox is being distributed in geographically diffuse sexual networks," the scientists wrote, adding that some of these connections extended beyond the UK.

"Most cases reported having sexual contact with new or casual partners, sometimes in the context of cruising grounds or chemsex," the experts said, referring to sex combined with drug use.

The expert noted that contact details for sexual partners were often unavailable.

Last month, a leading adviser to the World Health Organisation said the outbreak in Europe and beyond was likely spread by sex at two recent raves in Spain and Belgium.

Earlier this week, the WHO said more than 1000 cases of monkeypox had been reported in 29 countries that haven't previously had outbreaks of the smallpox-related disease, including the United States, Portugal, Australia, Spain, Italy and Canada.

Poland reported its first case on Friday.

No deaths have been reported.

The Health Security Agency found that many of the cases in the UK involved men who reported having sex in saunas, dark rooms or sex clubs.

"Therefore, collaborating with sex-on-premises venues to implement targeted interventions would support outbreak control," the agency said.

It added that using targeted messages on dating apps might also be useful or "support innovative approaches to contact tracing".

WHO chief Tedros Adhanom Ghebreyesus said it was likely monkeypox has been transmitting undetected for some time beyond Africa and that the United Nations health agency was concerned the disease might start to infect more vulnerable groups, such as pregnant women and children.

The WHO's top monkeypox expert, Dr Rosamund Lewis, said earlier this week there was still "a window of opportunity" to stop monkeypox from jumping into the general population and those more at risk of severe disease.

Most monkeypox patients experience only fever, body aches, chills and fatigue.

People with more serious illness may develop a rash and lesions on the face and hands that can spread to other parts of the body.

In severe cases involving people without access to health care, the WHO has noted a death rate of 3 to 6 per cent.

The ongoing outbreak of monkeypox in the UK and elsewhere marks the first time the disease has been known to spread among people who have no previous travel links to Africa.

UK scientists said they found three mutations in the monkeypox virus spreading in the country that they classed as "high priority" since they were found to worsen the disease in rats.

They said more research was needed to tell if the changes were significant.