States have continued to steadily lift restrictions, despite warnings from leading federal health officials like Dr. Anthony S. Fauci that the number of new coronavirus cases in the United States has risen to a very high level after its sharp decline has stalled and the country urgently needed to contain the spread of more communicable variants.

Arizona, California and South Carolina joined a growing list on Friday by relaxing restrictions to varying degrees. The Arizona governor ended capacity restrictions on businesses but said they must continue to need masks. The Republican governor of South Carolina lifted the state’s mask mandate in government buildings and recommended restaurants to continue to be masked.

In California, amusement parks, outdoor sports, and live stadium events can restart on April 1st with reduced capacity and mandatory masks.

“We have only just seen the worst of the climb,” said Dr. Fauci on Friday during a coronavirus briefing at the White House, adding that the country has plateaued between 60,000 and 70,000 new cases a day. “When you have that much viral activity on a plateau, it almost always means you are at risk for another spike.”

The 7-day average of new cases was 61,000 days on Friday, the lowest average since October, according to a New York Times database. But that number was still close to last summer’s highest peak.

The death toll is also falling, partly due to vaccinations in nursing homes. Still, the nation routinely reports 2,000 deaths in a single day.

Dr. Fauci warned the United States against taking the same treasonous path that Europe recently embarked on.

Updated

March 6, 2021, 6:57 p.m. ET

“You have reached a plateau,” he said. “And now, over the past week, they’ve seen cases rise 9 percent, which we really want to avoid.”

He warned that the virus mutates during replication, a process that can be prolonged if immunocompromised people are infected. He said there was an urgent need to maintain masking, hand washing and social distancing.

The B.1.1.7 variant, which was first identified in the UK, is spreading so rapidly in the US that data analysis suggests it will most likely account for 20 percent of new US cases as of this week. And scientists in Oregon have identified a single case of a native variant with the same spine as B.1.1.7 that has a mutation that could affect vaccine effectiveness.

Earlier this week, Texas and Mississippi, both Republican-led states, lifted mask mandates. President Biden denounced these moves as a “big mistake” reflecting “Neanderthal thinking” and said it was vital for officials to follow directions from doctors and public health executives when the coronavirus vaccination campaign begins Dynamism gains.

Other Republicans were more cautious. Ohio Governor Mike DeWine said he will lift all public health measures to contain the virus crisis, but only if new cases there fall below a certain threshold. In Alabama, Governor Kay Ivey said she would extend the state’s mask mandate through April 9.

In Arizona, Governor Doug Ducey has adopted what is known as a “measured approach,” which prevents local executives from taking actions that close businesses and allowing sports to be restarted in major leagues if approved by the state health department .

Among the Democrats, Michigan Governor Gretchen Whitmer said Tuesday that she would relax restrictions on businesses and allow family members who tested negative for the coronavirus to visit nursing home residents. In California, the state health department also relaxed some restrictions on Friday, stating that the amusement park could reopen on a limited basis as early as April 1.

In New York City, limited indoor dining has returned. And on Thursday, the Connecticut governor said the state would end capacity restrictions on restaurants, gyms and offices later this month. Masks remain required in both places.

Dr. Rochelle P. Walensky, director of the Centers for Disease and Prevention, has urged states not to relax their restrictions just yet. A new report from the CDC found that districts where restaurants in the US could be opened for personal meals saw an increase in daily infections weeks later. The study also said counties that issued mask mandates reported a decrease in virus cases and deaths within weeks.