Speaking at a hearing of the Senate health, education, labor and pensions committee with other members of the White House Coronavirus Task Force, Fauci said the country is "not in total control" of the pandemic and could see daily cases more than double from their current average.
"I can't make an accurate prediction, but it's going to be very disturbing," he said in a response to a question from Sen. Elizabeth Warren, D-Mass. "I will guarantee you that, because when you have an outbreak in one part of the country, even though in other parts of the country they're doing well, they are vulnerable.
"We are now having 40,000-plus new cases a day. I would not be surprised if we go up to 100,000 a day if this does not turn around, and so I am very concerned," he said.
The comments came after Fauci voiced "cautious optimism" that an effective vaccine against the novel coronavirus will be developed.
"Anyone who has been involved in vaccinations will tell you -- we'll have a safe and effective vaccine," he said, adding that researchers are generally "hopeful" that a coronavirus vaccine is possible and that the public will be receiving doses by next year.
"We are cautiously optimistic, looking at animal data and the preliminary data, that we will at least know the extent of the efficacy sometime in the winter and early part of next year," he said.
Fauci, however, said he's "quite concerned" about the recent spike in U.S. cases, and said there are likely two main causes -- that some states may have moved too quickly to reopen and too many Americans are ignoring health guidelines.
"We have got to get that message out that we are all in this together," he said. "If we are going to contain this, we've got to contain it together."
Federal and state officials have also blamed the surges on younger people failing to take precautions, like wearing face masks and practicing social distancing in public.
Also appearing before the panel Tuesday was Robert Redfield, director of the Centers for Disease Control and Prevention; Stephen Hahn, commissioner of the Food and Drug Administration; and Brett Giroir, assistant secretary of the Health and Human Services Department.
Redfield said it's "imperative" that Americans take personal responsibility in employing preventative measures to stop the virus' spread.
"Specifically, I'm addressing the younger members of our society, the millennials and the 'Generation Zs,'" he said.
"I ask those that are listening to spread the word," he added, urging younger people to "embrace the universal use of face coverings."
Tuesday's hearing comes after more than half of states reported new increases, which have prompted some to scale back plans to reopen businesses and ease restrictive safeguards.
Arizona Gov. Doug Ducey ordered Monday the closure of bars, gyms, movie theaters and water parks and Nevada Gov. Steve Sisolak postponed the second phase of his reopening plan until the end of July. Parts of Florida have closed beaches for the July Fourth weekend.
Tuesday was the second round of congressional testimony for Fauci, Redfield, Hahn and Giroir in as many weeks. A week ago, they each testified before the House energy and commerce committee. Fauci said then the United States had not planned to slow coronavirus testing, despite remarks to the contrary from President Donald Trump.