America could see as many as 100,000 new coronavirus cases a day if its current spikes in infections are not contained, the country's top infectious disease expert warned Congress on Tuesday.
Dr Anthony Fauci, a leading member of the White House's Coronavirus Taskforce, said the resurgence in severe outbreaks of the virus “puts the entire country at risk.”
He warned the country's daily number of new cases, already the highest in the world, could more than double if the situation is not brought under control.
"We are now having 40,000-plus new cases a day," Dr Fauci told US senators during a hearing on Tuesday. "I would not be surprised if we go up to 100,000 a day if this does not turn around. And so I'm very concerned because it could get very bad," he said.
The stark warning came as Dr Robert Redfield, director of America's Centers for Disease Control and Protection (CDC) told the hearing that vaccine scepticism among some Americans could pose a challenge to the country's attempt to contain the pandemic.
Dr Redfield said the CDC has spent three months drafting plans to rebuild "vaccine confidence".
The Trump administration aims to have 300 million doses of a vaccine by early next year.
However, Dr Fauci said "there is no guarantee" a safe and effective Covid-19 vaccine will be available in the near future as he urged Americans to come together and observe social distancing guidelines to contain the virus.
It comes as figures showed new virus cases in the US increased by 80 per cent in the last two weeks, according to analysis by the New York Times.
According to the New York Times database, 35 states have seen an increase in their daily cases over the last 14 days.
The increase has largely been attributed to spikes in the south and west of the country where states had pushed ahead with reopening. Four US states - Arizona, California, Florida and Texas - account for about half of the new infections.
The spikes have led state officials to dramatically reassess their decision to begin easing lockdown measures.
In Arizona, the Republican governor Doug Ducey ordered a new shutdown to combat the "brutal" surge in infections and hospitalisations hitting the state. Many businesses had been allowed to reopen in mid-May, but with daily cases regularly topping 3,000 in the last fortnight Mr Ducey ordered an immediate shut down of bars, cinemas, gyms and water parks for the next 30 days.
Los Angeles, which has become a new epicenter in the pandemic with nearly 3,000 new cases reported on Monday, announced a "hard pause" on plans to reopen entertainment venues such as cinemas and theme parks.
Meanwhile the city of Jacksonville in Florida, where Donald Trump plans to accept the Republican nomination in August, introduced an order requiring the public to wear face masks where social distancing is not possible.
Mr Trump has refused to wear a mask in public, even suggesting those who wear them do so to “signal disapproval of him.”
In a rare jibe from a member of the president's own party, Republican senator Lamar Alexander lamented that Mr Trump had made wearing a mask "part of the political debate".
Mr Alexander, chair of the Senate Health Committee, told the panel he had “suggested that the president occasionally wear a mask", even if he does not need to in most cases.
Joe Biden, the Democratic presidential nominee, also took aim at Mr Trump on Tuesday, accusing him of "historic mismanagement" of the pandemic.
Speaking in his hometown of Wilmington, Delaware, Mr Biden argued that earlier action by Mr Trump would have reduced the number who fell ill and the economic impact of the virus.
Courtney Parella, a spokeswoman for the Trump campaign, said Mr Biden was "fearmongering and rooting against America’s success" while the president enacted measures that had slowed the spread of the virus.