The federal government warned impatient governors Friday of easing pandemic control measures.Other cases of new variants have been found and may suggest a return to normal is not quite as close as many Americans had hoped.

“Things are difficult,” said Rochelle Walensky, director of the Centers for Disease Control and Prevention, at a White House briefing on the pandemic. “Now is not the time to relax restrictions.”

Your warning was given by Dr. Anthony S. Fauci, the government’s leading epidemiologist, supported as the Biden government struggled to stay one step ahead of each new wave. President Biden himself flew to Houston to showcase the government’s newest mass vaccination site.

According to a New York Times database, virus cases in the United States appear to be leveling off from the steep decline that began in January. The numbers are comparable to those reported at the end of October. Cases have increased slightly from week to week in the past few days, despite severe weather restricting testing and reporting in Texas and other states over the past week and not all states reporting full dates for the Presidents Day holidays. The seven-day average of new cases was 77,800 on Thursday.

While deaths tend to fluctuate more than cases and hospital admissions, Dr. Walensky at the meeting on Friday that the most recent average of seven days was slightly higher than the average at the beginning of the week. The seven-day average of newly reported deaths was 2,165 on Thursday.

“We at CDC view this as a very worrying shift in trajectory,” she said, adding, “I want to be clear: cases, hospital admissions and deaths – all remain very high and the recent shift in the pandemic must be taken extremely seriously.”

Dr. Walensky said some of the increase could be due to new variants of the coronavirus that are spreading more efficiently and faster. The so-called B.1.1.7 variant, which came onto the market for the first time in Great Britain, now accounts for around 10 percent of all cases in the USA, a few weeks ago it was one to four percent. The US’s ability to track variants is much less robust than the UK’s.

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Apr. 26, 2021, 5:14 p.m. ET

“I know people are tired. You want to get back to life, to normal, ”she said. “But we’re not there yet.”

As cases receded, some governors in the United States have started easing restrictions on pandemics. States with Republican governors seemed more willing to face setbacks, though New York, which has a Democrat governor, has also eased restrictions on a variety of activities.

On Friday, Republican Governor Henry McMaster of South Carolina announced that restaurants could serve alcohol on Mondays after 11 p.m. and residents would not need state approval to hold events with 250 or more people. To limit the spread of the virus, the state last year ordered bars to stop drinking after 11 p.m., which is three hours earlier than the nightly bar crowd was used to.

Brian Symmes, a spokesman for Mr. McMaster, said the governor “valued perspectives different from his own” but “respectfully disagreed” with Dr. Walensky’s assessment.

On Thursday, Texas Governor Greg Abbott said he was considering lifting a nationwide mask mandate that began in July.

In Mississippi, Governor Tate Reeves said he was also considering lifting some restrictions, particularly mask mandates for people who have been fully vaccinated. As of Friday, 13 percent of the state’s population had received at least one shot and 6.2 percent had received two, according to a Times database.

Dr. Fauci repeated Dr. Walensky suggested that more state or local rollbacks would be unwise, noting that the cases remain in a “very precarious position.”

“We don’t want to be people who always look at the dark side of things, but you want to be realistic,” he said. “So we need to look closely at what happens to these numbers over the next few weeks before you understandably begin to relax certain restrictions.”

Eileen Sullivan Remy Tumin, Nicholas Bogel-Burroughs and Mitch Smith contributed to the coverage.