The National Institute of Allergy and Infectious Diseases director Dr Anthony Fauci has predicted the US could have 'millions' of Covid-19 cases and '100,000 to 200,000' deaths as the pandemic escalates across the country.
Currently the US has reported more than 124,000 confirmed cases and over 2,100 deaths, a number that doubled in just two days.
Dr Fauci appeared on CNN's State of the Union on Sunday to discuss the federal response to the coronavirus and whether restrictions would be lifted this week when the 15-day period would be up.
While the expert said he currently anticipated the numbers he referred to, he said he did not want to be held down by them because the pandemic was "such a moving target".
President Donald Trump and his administration has said it is considering opening up parts of the country not impacted as hard by the coronavirus to help the US economy and job market.
Testing across the country would need to increase, though, to determine if an area is appropriate for reduced restrictions.
Mr Fauci said he would support the rollbacks of regulations and social-distancing guidelines on parts of the US depending on what the testing looked like in those areas.
As of right now, he acknowledged "it's a little iffy there".
"It still is not a perfect situation," he said about testing.
The expert, who is a member of the Trump administration's coronavirus task force, was also asked about the president's threat on Saturday to quarantine off hot-spot locations, such as New York, New Jersey, and Connecticut.
It was decided instead to issue a strict travel advisory for residents in those three states after officials "had very intensive discussions last night" with the president at the White House.
"What was trying to be done is to get people, unless there's necessary travel, so all nonessential travel, just hold off, because what you don't want is people travelling from that area to other areas of the country and inadvertently and innocently infecting other individuals," Mr Fauci said.
"We felt the better way to do this would be an advisory as opposed to a very strict quarantine. And the president agreed, and that's why he made that determination last night," he added.
The travel advisory was issued by the Centres for Disease Control and Prevention.
Mr Fauci added about 56 per cent of confirmed cases in the US come from New York City.
The total number of confirmed cases rose to 53,455 and 883 deaths. Of those cases, more than 30,765 come from New York City.
States including Texas and Rhode Island issued travel advisories against people coming from New York last week. Anyone travelling from that area must enter into a 14-day self-quarantine.