National Institute of Allergy and Infectious Disease Director Dr. Anthony Fauci told the "Fox News Rundown" Tuesday that it is still "too early to say" what has caused a major increase in the number of coronavirus cases in parts of the U.S., particularly the South and West.

"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."

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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, for example, 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.

CLEVELAND CORONAVIRUS SPIKE COMES AFTER MAY PROTEST

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.

"If you're in a crowd and you're demonstrating and you don't have a mask, you're at risk, whether or not that's going to show up on a chart ... It may take three, four weeks to show up ..." said Fauci, who added that "masks diminish the risk, but if you put yourself in a situation with a crowd, it doesn't completely eliminate the risk. So, I don't think it's accurate to say that people congregated together and we didn't see any outbreak. Well, you know, many of those people in those demonstrations were from different locations than where the demonstration took place."

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According to Fauci, part of the complexity of fighting the pandemic in the United States is "the nature of our country being a very large country, that's very heterogeneous and it is very difficult to get the message across, particularly [with] the independent spirit of the relationship between the federal government and the states. That's not an excuse, it's just the reality of the United States.

"When Europe -- when Italy shut down, when Japan shut down, 95 percent of the country shut down. When we went into the so-called lockdown, about 50 percent or less actually really locked down."

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