The BA.2 sub-variant of Omicron was estimated to be 23.1 percent of the coronavirus variants circulating in the United States as of March 12, the U.S. Centers for Disease Control and Prevention (CDC) said on Tuesday.

Scientists are tracking a rise in cases caused by BA.2, which is spreading rapidly in parts of Asia and Europe.

According to the CDC's data, the sub-variant now makes up 39 percent of total cases in regions including New Jersey, New York, Puerto Rico and Virgin Islands.

In states such as Connecticut, Maine, Massachusetts, New Hampshire, Vermont and Rhode Island, the sub-variant now makes up about 38.6 percent of total cases.

U.S. daily cases have started to decline in recent weeks after touching record levels in January, with the CDC easing its COVID-19 guidelines for masks, including in schools.

Initial data for the BA.2 sub-variant, which has begun to replace Omicron's more common BA.1 variant, shows no significant difference in disease severity, the World Health Organization in a statement said last month.

Other Omicron sub-variants that have been circulating since December, called BA.1.1 and B.1.1.529, now make up for around 66.1 percent and 10.8 percent of circulating variants, respectively.

For the week ending March 5, the CDC estimates that BA.2 made up 13.7 percent of circulating variants, revised up from 11.6 percent, according to a CDC model that estimates proportions of circulating variants. The agency has revised its estimates in the past as it gets more data.