The World Health Organisation says "sustained transmission" of monkeypox worldwide could result in the virus beginning to move into high-risk groups like pregnant women, immunocompromised people and children.

The WHO said on Wednesday it is investigating reports of infected children, including two cases in the United Kingdom as well as following up reports in Spain and France.

None of the cases in children have been severe.

The virus has now been identified in more than 50 new countries outside the countries in central and west Africa where it is endemic.

The WHO says cases are also rising in those countries, calling for testing to be ramped up.

"I'm concerned about sustained transmission because it would suggest that the virus (is) establishing itself and it could move into high risk groups including children, the immunocompromised and pregnant women," WHO chief Tedros Adhanom Ghebreyesus said in an online briefing from Geneva on Wednesday.

There have been more than 3400 cases of monkeypox, and one death, since the outbreak began in May, largely in Europe among men who have sex with men, according to a WHO tally.

There have also been more than 1500 cases and 66 deaths in countries this year where it more usually spreads.

Last week, the WHO ruled that the outbreak did not yet represent a public health emergency, its highest level of alert.

However, Tedros said the WHO was tracking the outbreak closely and would reconvene the committee "as soon as possible" to assess whether this was still the case.

Tedros said 11 of the experts at the last committee meeting were against declaring an emergency while three were in favour.

The United Nations agency said it was also working on a mechanism to distribute vaccines more equitably, after countries including the UK and the US suggested they were willing to share their stockpiled smallpox vaccines, which also protect against monkeypox.