The United States recorded an all-time daily high of 40,000 coronavirus infections on Thursday, figures from Johns Hopkins University (JHU) show.
A recent surge in infections and hospitalisations has prompted the states of Texas, Florida and Arizona to pause reopening plans.
JHU's previous high of 36,400 was on 24 April when less testing took place.
The US has 2.4 million confirmed infections and 122,370 deaths – more than any other country.
While some of the increase in daily cases recorded is down to increased testing, the rate of positive tests in some areas is also increasing.
Health officials in the US estimate the true number of cases is likely to be 10 times higher than the reported figure.
The Centers for Disease Control (CDC) said up to 20 million Americans may have been infected with coronavirus. The estimate was based on blood samples collected from across the country which were tested for the presence of antibodies to the virus.
The surge in cases was being driven by young people testing positive, especially in the south and west of the US, said the head of the CDC, Dr Robert Redfield.
Which states are worst hit?
Texas, which has been at the forefront of moves to end lockdown measures, has seen thousands of new cases, prompting Republican Governor Greg Abbott to call a temporary halt to its reopening.
"This temporary pause will help our state corral the spread until we can safely enter the next phase of opening our state for business," he said on Thursday.
- Texas confirmed a record 5,996 new cases on Thursday
- There were also 47 new deaths, the highest daily toll for a month
- The state has also seen a record number of people requiring hospital treatment for 13 days in a row
- Elective surgery has been suspended in the Houston, Dallas, Austin and San Antonio areas to free up beds
- More than 10% of the tests carried out over the past week have come back positive
- All but 12 of the state's 254 counties have reported cases
Florida's governor said there was no plan to continue reopening step-by-step. "We are where we are. I didn't say we were going to go on to the next phase," Ron DeSantis told reporters on Thursday.
Arizona has emerged as another epicentre of the crisis. Disease trackers there say the state has "lost control of the epidemic", the Washington Post reports. Governor Doug Ducey, who had been giving businesses a "green light" to reopen, now says Arizona residents are "safer at home".
The light is at "yellow", Gov Ducey said on Thursday. "I'm asking for Arizonans to proceed with caution, to go slower, to look both ways."
Other states, including Alabama, California, Idaho, Mississippi, Missouri, Nevada, Oklahoma, South Carolina and Wyoming, have all seen record daily increases in the number of confirmed cases this week.