World Health Organization (WHO) authorities Tuesday urges countries to advise travelers to wear masks on long-haul flights to avoid the fast spread of the new Omicron sub-variant of COVID-19.

WHO and Europe authorities held a press conference and said the XBB.1.5 sub-variant is being found in Europe in tiny but increasing numbers.

Passengers should be recommended to wear masks in high-risk circumstances such as long-haul flights, said the WHO's lead emergency officer for Europe, Catherine Smallwood, adding: "this should be an advice made to passengers arriving from anywhere where there is widespread COVID-19 transmission".

According to US health officials, the most transmissible Omicron sub-variant discovered thus far, XBB.1.5, accounted for 27.6% of COVID-19 cases in the United States for the week ending January 7.

According to specialists, current immunizations continue to protect against severe symptoms, hospitalization, and death.

"Countries must consider the evidence foundation for pre-departure testing," Smallwood noted, emphasizing the need of not focusing solely on one geographic area.