National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci on Tuesday warned that the U.S could see as many as 100,000 coronavirus cases a day if a current spike in cases does not subside.

“We are now having 40-plus-thousand new cases a day, I wouldn't be surprised if we go up to 100,000 [cases] a day if this does not turn around,” he said at a Senate Health, Education, Labor, and Pensions Committee hearing. “I am very concerned."

FAUCI: 'TOO EARLY TO SAY' WHAT CAUSED CORONAVIRUS CASES TO SURGE IN PARTS OF US 

There has been an increase in cases, particularly in the South and the West. It comes after a month that has seen states reopening their economies, including bars and restaurants, and has seen mass protests and riots in the wake of the death of George Floyd.

States, including Arizona, California, Florida and Texas, have paused or backtracked their reopening plans as the number of confirmed coronavirus cases has soared. In Arizona, health officials reported 3,858 more confirmed coronavirus cases Sunday, the most reported in a single day in the state so far and the seventh time in the past 10 days that daily cases surpassed the 3,000 mark.

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Earlier, Fauci told the "Fox News Rundown" that it was "too early to say" what has caused the spike.

"A lot of people came out [on] Memorial Day, which is about four weeks ago, three-and-a-half to four weeks ago, and were out on the beaches and in the bars," Fauci told host Jessica Rosenthal. "Right now, we're seeing the result of that in Florida and in Texas and in certain other locations."

A large share of the cases are among young people who are going out again to bars and restaurants, who are less likely to suffer serious symptoms from the virus. But it has raised fears that young people could go on to infect the elderly or vulnerable.

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Meanwhile, Cleveland's public health director revealed Monday that the current spike in coronavirus cases there is rooted in the days following a May 30 Black Lives Matter protest, when the city lifted daytime travel restrictions.

Health experts have long warned about the possibility of a second wave of the virus as the country re-opens from the harsh lockdowns that were put into place in March in response to the virus.

But efforts to re-impose lockdown restrictions would likely face significant opposition from those who question the efficacy of such a strategy, and who also highlight the economic and social damage that lockdowns have caused.

Fox News' Samuel Chamberlain and The Associated Press contributed to this report.