WASHINGTON — The fast-spreading Omicron variant is now the main coronavirus strain in the United States, accounting for 73.2 percent of new cases over the past week for which data is available, health authorities reported Monday.

The spike, which was tallied by the Centers for Disease Control and Prevention, is based on data for the week ending on Saturday. During the prior week period, Omicron accounted for only 12.6 percent of US cases.

Omicron already constitutes more than 90 percent of new US cases in the Pacific Northwest and much of the US South and parts of the Midwest, the CDC indicated.

The news comes ahead of a speech by US President Joe Biden on Covid on Tuesday. White House press secretary Jen Psaki has already said he does not plan on “locking the country down” in response to the surge.

“This is a speech outlining and being direct and clear with the American people about the benefits of being vaccinated, the steps we’re going to take to increase access and to increase testing.”

Top US pandemic advisor Anthony Fauci warned on Sunday of a bleak winter ahead as the Omicron coronavirus variant spurs a new wave of infections globally.

“With Omicron,” Fauci told NBC News, “it is going to be a tough few weeks to months as we get deeper into the winter.”

Despite indications that Omicron is not more severe than the still-dominant Delta variant, early data suggests it could be more infectious and possibly have higher resistance to vaccines.

Since it was first reported in South Africa in November, Omicron has been identified in dozens of countries, dashing hopes that the worst of the pandemic is over.

Across the United States, hospitals are getting busy, testing centers are seeing long lines, and sports and entertainment events are being cancelled.

Getting the virus under control has proven difficult in a country where vaccination and mask-wearing have become divisive political issues, and federal mandates end up in protracted legal battles.