Top US disease expert Dr Anthony Fauci has told the US Senate that he "would not be surprised" if new virus cases in the country reach 100,000 per day.
"Clearly we are not in control right now," he testified, warning that not enough Americans are wearing masks or social distancing.
During the hearing on Tuesday, he said about half of all new cases come from four states.
Earlier, the New York governor said nearly half of all Americans must self-quarantine if they visit the state.
On Tuesday, cases rose by more than 40,000 in one day for the fourth time in the past five days.
The surge - which is occurring particularly strongly in southern and western states - has forced at least 16 states to pause or reverse their reopening plans, according to CNN. Florida, Arizona, Texas and California are the four states referenced by Dr Fauci as being most heavily hit currently.
What did Dr Fauci say?
Testifying to a Senate committee on the effort to reopen schools and businesses, the director of the National Institute of Allergy and Infectious Diseases criticised states for "skipping over" benchmarks required for reopening, and said cases will rise as a result.
"I can't make an accurate prediction, but it is going to be very disturbing, I will guarantee you that," he told Senator Elizabeth Warren, reports BBC.
"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 can't just focus on those areas that are having the surge. It puts the entire country at risk," he added.
Dr Fauci also called on the US government to produce face masks to be distributed for free to all Americans, and condemned the "all or none phenomenon" of some people who have completely disregarded social distancing measures.
'Herd immunity unlikely'
Dr Fauci warned on Monday that the US is "unlikely" to develop herd immunity to the coronavirus even once a vaccine is available, which he earlier predicted could happen by early 2021.
He said this was due to the combination of a vaccine that is potentially only partially effective, and the large number of Americans who might refuse to get it.
"There is a general anti-science, anti-authority, anti-vaccine feeling among some people in this country - an alarmingly large percentage of people, relatively speaking," he said, calling for more education to promote confidence in vaccines.
Dr Fauci added that he would gladly "settle" for a vaccine that is only 70% to 75% effective at first.
According to the CDC website, herd immunity is reached when "a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely".