The World Health Organization (WHO) said that Omicron variant of COVID-19 could be more infectious than the original version and has been detected in 57 countries.

The UN health agency said that the heavily mutated variant has rapidly become the dominant variant worldwide since it was first detected in Southern Africa 10 weeks ago.

WHO, in its weekly epidemiological update said that the variant, which accounts for over 93 percent of all coronavirus specimens collected in the past month, counts several sub-lineages: BA.1, BA.1.1, BA.2 and BA.3.

The BA.1 and BA.1.1 -- the first versions identified -- still account for over 96 percent of all the Omicron sequences uploaded to the GISAID global science initiative, the health agency said.

But there has been a clear rise in cases involving BA.2, which counts several different mutations from the original -- including on the spike protein that dots the virus's surface and is key to entering human cells.

"BA.2- designated sequences have been submitted to GISAID from 57 countries to date," WHO said, adding that in some countries, the sub-variant now accounted for over half of all Omicron sequences gathered.

The UN health agency said little was known yet about the differences between the sub-variants, and called for studies into its characteristics, including its transmissibility, how good it is at dodging immune protections and its virulence.

Several recent studies have hinted that BA.2 is more infectious than the original Omicron.

Maria Van Kerkhove, one of the WHO's top experts on Covid, told reporters Tuesday that information about the sub-variant was very limited, but that some initial data indicated BA.2 had "a slight increase in growth rate over BA.1", reported NDTV.

Omicron in general is known to cause less severe disease than previous coronavirus variants like Delta, and Van Kerkhove said there so far was "no indication that there is a change in severity" in the BA.2 sub-variant.