The UK Health Security Agency has detected four new cases of monkeypox — three in London and one linked case in Newcastle, northeast England — after registering three cases earlier in May.
The World Health Organization (WHO) has said it was coordinating with UK and other European health officials after British authorities detected at least seven cases of monkeypox this month.
In a briefing on Tuesday, the WHO said there was also one "additional probable" case reported in Britain.
Maria Van Kerkhove, another WHO official, said the organisation was working with its regional office, as well as the European Centers for Disease Control and the UKHSA, to better understand the outbreak.
"We are working very closely with [them]... to evaluate each of these cases, the source of their infection," she added.
Van Kerkhove noted that involved "forward contact tracing to ensure that there isn't further human-to-human transmission, as well as back contact tracing to better understand the source of their infection".
Ibrahima Soce Fall, assistant director-general for emergency response at the WHO told reporters that sexual contact is likely to be the main reason behind the spread of monkeypox in the UK.
"(It) is new information we need to investigate properly, to understand better the dynamic of local transmission in UK and in some other countries," said Fall.
Symptoms in humans of monkeypox — which is endemic in parts of Central and Western Africa — include lesions, fever, muscle ache and chills.
Transmission is usually via close contact with infected animals such as rodents and monkeys and is limited between people. It has only been fatal in rare cases.
The UK Health Security Agency (UKHSA) said on Monday it had detected four new cases.
Those patients needing medical care are in specialist infectious disease units at hospitals in London and Newcastle, according to the agency.
None have known connections with the three earlier confirmed cases, the first of which was linked to travel from Nigeria, it added.