The new coronavirus variant Omicron accounts for 73.2 percent of U.S. coronavirus infections based on sequencing data for the week ended December 18, the U.S. Centers for Disease Control and Prevention said on Monday.

The health authorities said the fast-spreading variant is now the main coronavirus strain in the U.S.

The newly released data was compared to 12.6 percent from the week ending December 11. In the week ending December 4, Omicron only accounted for 0.7 percent of all infection cases.

The U.S. detected its first Omicron variant in California on December 1. The variant has spread to at least 48 states as of Monday.

Late on Monday, U.S. President Joe Biden warned that Omicron cases will increase in the coming days, even among fully vaccinated individuals. However, he still urged people to get the shot.

"We know that vaccines are working. If you are boosted with Pfizer or Moderna, you have a high degree of protection against severe illness with Omicron. If you're an adult choosing to be unvaccinated, you will face an extremely difficult winter for your family and community," he tweeted.

(With input from agencies)