For the third consecutive day, the US set a record for number of new coronavirus cases.
The new infections reported on Wednesday topped 50,000, with 40 states reporting increases, taking the US total to more than 2.7 million, with almost 129,000 deaths.
Severe outbreaks have emerged in Arizona, Texas, Florida and California.
In 20 states, more than 5 per cent of the tests conducted returned positive results.
A reluctance to wear masks, backed by US President Donald Trump, and flouting of social-distance rules is partly blamed for the new outbreak ravaging the southern and western states.
Executive orders have been announced in several states, including Texas and Kansas, to mandate face coverings in public.
“In the past few weeks, there has been a swift and substantial spike in coronavirus cases,” Texas Governor Greg Abbott said.
Florida reported more than 10,000 new cases for the first time on Thursday, six times higher than the daily count from less than a month ago.
The state also reported 67 deaths for the second time in a week and 325 patients admitted to hospital, one of the biggest 24-hour jumps in Florida yet.
“I’m discouraged because we didn’t act fast enough to shut things down and we could have done a much better job getting a handle on the virus,” said Megan Archer, 39.
Ms Archer, from West Palm, lost her job with a county parks department during the outbreak.
By the end of the month, potential Covid-19 vaccines will enter late-stage clinical studies, with others starting in August, September and October, the US government's top infectious diseases expert said on Thursday.