Top disease researcher Dr Anthony Fauci has told the US Senate that he "wouldn't be surprised" if new virus cases in the country reach 100,000 per day.
"Obviously, we don't have control right now," he testified, warning that not enough Americans are wearing masks or social distancing.
During the hearing, he said about half of all new cases come from four states.
Earlier, the governor of New York said that almost half of all Americans must quarantine themselves if they visit the state.
On Tuesday, cases increased by more than 40,000 in one day for the fourth time in the last five days.
The surge – which is particularly strong 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 referred by Dr. Fauci to be the hardest hit for the time being.
For some, the new measures come over a month after they first started opening their economies again.
Also on Tuesday, New York Governor Andrew Cuomo expanded the number of Americans required to self-quarantine for 14 days if they visit the state. There are now 16 states on the list.
The recently added states are California, Georgia, Iowa, Idaho, Louisiana, Mississippi, Nevada and Tennessee.
They join Alabama, Arkansas, Arizona, Florida, North Carolina, South Carolina, Texas and Utah.
Overall, the order affects 48% of all US residents, according to a USA Today analysis.
What did Dr. Fauci say?
The director of the National Institute of Allergy and Infectious Diseases, who testified before a Senate committee on the reopening of schools and businesses, criticized states for "skipping" standards required for reopening, saying the cases would arise as a result.
"I can't make an accurate prediction, but it's going to be very disturbing, I guarantee you," he told Senator Elizabeth Warren.
"Because when you have an outbreak in one part of the country, even if in other parts of the country it is good, they are vulnerable."
"We can't just focus on the areas that are growing sharply. It puts the whole country at risk," he added.
Dr. Fauci also asked the US government to produce face masks to be distributed free to all Americans, condemning "all or no phenomenon" to any people who have completely disregarded social distance measures.
Robert Redfield, director of the Centers for Disease Control and Prevention (CDC) – who also testified on Tuesday – told lawmakers that 12 states have seen hospitalization levels rise, and Arizona has recorded a rising death rate.
"It is crucial that we all take personal responsibility for slowing down the transmission of Covid-19 and embracing the universal use of facial coatings," said Dr. Redfield.
"The disease affects us all, and it will take everything to work together to stop it."
Before the hearing began, Republican Senator Lamar Alexander, who chairs the committee, appealed to President Donald Trump to wear a face mask – an act he has not yet done in a public event.
"Unfortunately, this simple life-saving practice has become part of the political debate," the ally told Trump.
"If you are for Trump, you are not wearing a mask, if you are against Trump, you will," he continued, continuing to "suggest that the President occasionally wear a mask."
"The president has plenty of admirers who will follow his lead," he said.
"We have too much virus"
On Monday, Dr. Anne Schuchat, chief executive of CDC, warned that the United States is not responding like other countries that have shown success in containing coronaviruses and has allowed the virus to spread much more widely and rapidly.
"We are not in the situation of New Zealand or Singapore or Korea where a new case is quickly identified and all contacts are tracked and people are isolated who are sick and people who are exposed are quarantined and they can keep things under control," said Schuchat in an interview with the Journal of the American Medical Association.
"We have far too much virus across the country for it right now, so it's very disheartening."
New Zealand declared the country infectious on June 8, and has since then had to contain several cases that came from international travelers. South Korea has been aggressively employing contact trackers, and has registered fewer than 100 cases per day since April 1. Singapore's outbreak peaked in mid-April when 1,400 new cases were reported in one day.
The United States has registered 2,682,877 confirmed cases of coronavirus as of Tuesday, according to Johns Hopkins University, and has a total of 129,544 deaths.
"This is really the beginning," Schuchat added.
"I think there was a lot of wishful thinking around the country that & # 39; hey it's summer & # 39; It's all going to go well. We're over this and we're not even starting to be over this. There's a lot of worrying factors about the last week or so. "
& # 39; Herd immunity unlikely & # 39;
Dr. Fauci warned Monday that the United States is "unlikely" to develop herd immunity to the coronavirus even when a vaccine is available, which he predicted could happen in 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 may refuse to receive it.
"There is a general anti-science, antitrust, and vaccination sentiment in some people in this country – a worryingly large proportion of people, relatively speaking," he said, calling for more education to promote vaccine confidence.
Dr. Fauci added that he would like to "settle" with a vaccine that is only 70% to 75% effective in the beginning.
According to the CDC website, immunity is achieved when "a sufficient proportion of the population is immune to an infectious disease (through vaccination and / or past disease) to make its spread from person to person unlikely".