Dr Anthony Fauci has said coronavirus cases could grow to 100,000 a day in the US if Americans do not start following public health recommendations.
The nation's leading infectious disease expert made the remark at a Senate hearing on reopening schools and workplaces.
Asked to forecast the outcome of recent surges in some states, Dr Fauci said he cannot make an accurate prediction, but believes it will be "very disturbing".
"We are now having forty-plus-thousand new cases a day. I would not be surprised if we go up to to 100,000 a day if this does not turn around, and so I am very concerned," said Dr Fauci, infectious disease chief at the National Institutes of Health.
#COVID19 is an emerging, rapidly evolving situation. Learn about the basics like symptoms & what to do if you are sick; and see the latest info from @CDCgov on #NIH's page: https://t.co/9aGQF3GG9p#coronavirus
- NIH (@NIH) March 17, 2020
Dr Fauci said areas seeing recent outbreaks are putting the entire nation at risk, including areas that have made progress in reducing Covid-19 cases.
He cited recent video footage of people socialising in crowds, often without masks, and otherwise ignoring safety guidelines.