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.
Moderna, the company at the forefront of the country's vaccine development attempts, said earlier in the day that a late-stage trial with 30,000 volunteers would begin this month.
"We may be able to at least know whether we are dealing with a safe and effective vaccine by the early winter, late winter or the beginning of 2021," Dr Anthony Fauci told Jama Network.
Meanwhile, US unemployment fell to 11 per cent in June as the economy added 4.8 million jobs, the government reported on Thursday.
But any recovery of the job market may already be faltering because of new closures and layoffs triggered by the resurgence of the coronavirus.
While the jobless rate was down from 13.3 per cent in May, it is still at a Depression-era level.
And the data was gathered during the second week of June, just before states began to reverse or suspend reopening their economies.
"This is a bit of a dated snapshot at this point,” said Jesse Edgerton, an economist at JPMorgan Chase.
