The new XBB.1.5 variant of COVID is the "most transmissible" yet detected, a senior World Health Organization (WHO) official has warned.
The variant, which is a sub-variant of Omicron, is confirmed to be spreading in 25 countries, including the UK and US.
Dr Maria Van Kerkhove, the WHO's COVID-19 technical lead, told a press conference on Wednesday: "It is the most transmissible sub variant that has been detected yet.
"The reason for this are the mutations that are within... this sub variant of Omicron, allowing this virus to adhere to the cell and replicate easily.
"And we are concerned about its growth advantage, in particular in some countries in Europe and in the US in North America, particularly the northeast part of the United States where XBB.1.5 has rapidly replaced other circulating variants."
Watch: WHO 'really concerned' about COVID picture with 'intense transmission in several regions'
Over 40% of COVID-19 cases in the US are now caused by XBB.1.5, data from the Centers for Disease Control and Prevention showed on Friday, with the subvariant doubling from the previous week.
The XBB variant has also been driving up cases in parts of Asia, including Singapore.
Dr Van Kerkhove said there was so far no evidence to suggest the variant caused more severe illness but added the more the new strain circulated, the more likely it would change.
It came as the WHO said China's COVID-19 data was not giving an accurate picture of the situation there and underrepresented hospitalisations and deaths from the disease.
The UN agency was preparing to meet Chinese scientists again on Thursday as part of a wider briefing among member states on the global COVID-19 situation as concerns grow about the rapid spread of the virus in China.
Mike Ryan, WHO's emergencies director, told a briefing in Geneva: "We believe the current numbers being published from China underrepresent the true impact of the disease in terms of hospital admissions, in terms of ICU admissions, particularly in terms of death."
He added the WHO believed the Chinese government's definition for death is "too narrow".
Late last month, China narrowed its definition for classifying deaths as COVID-related, counting only those involving COVID-caused pneumonia or respiratory failure, raising eyebrows among world health experts.
The WHO says deaths should be attributed to COVID-19 if they result from a "clinically compatible illness" in a patient with a probable or confirmed infection, and no other unrelated cause of death - like trauma - is involved.
Ryan added: "We still do not have complete data."