According to new data from the Centers for Disease Control and Prevention, the proportion of coronavirus cases in the United States caused by the Omicron variant has increased dramatically, and a significant surge in infections could occur as soon as next month.

According to agency projections released on Tuesday, Omicron accounted for 2.9 percent of cases across the country during the week ending on Saturday, up from 0.4 percent the previous week.

Omicron infections had already reached 13.1 percent in the region comprised of New York, New Jersey, Puerto Rico, and the United States Virgin Islands.

C.D.C. officials warned of two possible scenarios during a briefing on Tuesday with state and local health officials and representatives from public health labs across the country. The first was a tidal wave of Omicron and Delta infections that could arrive as early as next month, just as influenza and other winter respiratory infections were peaking.

“The early indicators indicate that there will be waves,” said Scott Becker, chief executive officer of the Association of Public Health Laboratories, who was on the call.

“We are already expecting an increase, simply because we saw a lot of respiratory viruses this fall, including R.S.V., which was widespread,” he added.

Officials from the federal government also proposed a second scenario in which there is a smaller increase in Omicron cases in the spring. It was difficult to tell which forecast was more likely.

Early evidence of the variant has only recently emerged, and it is unclear how frequently Omicron infections result in hospitalizations or deaths. The variant appears to be able to evade the body’s immune defenses in part, but scientists have yet to determine to what extent vaccination and prior infection may protect individuals from severe disease.

To track variants, the Centers for Disease Control and Prevention (CDC) employs a national surveillance system that collects samples as well as genetic sequences generated by commercial laboratories, academic laboratories, and state and local public health laboratories.

The U.S. system was relatively slow to pick up cases of the variant, perhaps in part because of travel patterns or restrictive U.S. entrance rules. However, the system is also limited by blind spots and delays.

The Centers for Disease Control and Prevention reported last week that of the 43 known infections detected in the United States in the first eight days of December, 34 of the patients, or 79 percent, had been fully vaccinated when they first showed symptoms or tested positive. Only about a third of the 43 people had traveled internationally in the two weeks preceding diagnosis, indicating that the variant had spread within the community.

According to Xavier Becerra, the secretary of health and human services, the fight against Omicron may necessitate the federal government replenishing response funding. Mr. Becerra told reporters that about $10 billion of the $50 billion Congress had set aside for testing remained.

Staffing may be a challenge for public health labs, as it is for hospitals, according to Mr. Becker.

Health officials in Europe have warned of an increase in Omicron cases. According to Monday’s estimates, cases of the variant were doubling every two days in Denmark, which is similar to the United States in terms of vaccination rates and average age. The World Health Organization’s director general, Dr. Tedros Adhanom Ghebreyesus, stated in a press conference on Tuesday that “Omicron is spreading at a rate we have not seen with any previous variant.”

Dr. Tedros and senior World Health Organization officials warned against underestimating the variant. “Even if Omicron causes fewer severe cases, the sheer number of cases could once again overwhelm underprepared health-care systems,” he said.

In the United States, state and local health officials urged residents to get vaccinated, get booster shots, and wear masks in public indoor settings to help prevent the spread of Covid. Families and friends gathering for the holidays should be tested before celebrating together, preferably outside or in well-ventilated areas.

“As the Delta variant continues its rapid spread in the U.S., state and territorial health leaders are becoming increasingly concerned about emerging data from Europe and South Africa that indicate the Omicron variant may be even more transmissible,” said Michael Fraser, chief executive officer of the Association of State and Territorial Health Officials.