The Centers for Disease Control (CDC) says the true number of cases is likely to be 10 times higher than the reported figure.
It comes as the state of Texas halted its reopening as infections and hospitalisations surged.
The US has recorded 2.4m confirmed infections and 122,370 deaths.
Some southern and western states have been reporting record numbers of cases in recent days.
The University of Washington predicts 180,000 US deaths by October - or 146,000 if 95% of Americans wear masks.
Photo BBC/Reuters Caption: Texans queue for tests in their cars - more than 10% of tests came back positive over the past week