The Centers for Disease Control and Prevention this week said the actual number of infections is likely two to 13 times higher than the reported figure.
Caseloads are rising in southern and western states, including Florida, Texas and California, which on Wednesday reported a record number of new daily infections and overtook New York as the state with the most confirmed cases.
The numbers of daily hospitalisations and deaths, which experts say lag behind the trend in cases, have also been increasing.
Between 60,000 to 77,000 new cases have been recorded daily in the past two weeks across the country.
More than 1000 deaths were reported on both Tuesday and Wednesday.
President Donald Trump, who has come under fire for downplaying the crisis and faces sinking poll numbers over his response, conceded earlier this week that the US outbreak would “get worse before it gets better”.