At least 20 million people in the US may already have been infected with COVID-19, according to the latest estimate by health officials.
The Centres for Disease Control (CDC) says the true number of cases is likely to be 10 times higher than the 2.3 million cases that have been confirmed.
The advisory comes as the US registered a near-record number of new coronavirus cases per day while an outbreak in China’s capital appears to have been brought under control.
The crisis has deepened in Arizona, and the governor of Texas has began to backtrack on reopening, as the daily number of confirmed cases across the US closed in on the peak reached during the dark days of April.
While greatly expanded testing probably accounts for some of the increase, experts say other measures indicate the virus is making a comeback.
Daily deaths, hospitalisations and the percentage of tests that are coming back positive have also been rising over the past few weeks in parts of the country, mostly in the south and west.
In Arizona, 23 per cent of tests conducted over the past seven days have been positive, nearly triple the national average, and a record 415 patients were on ventilators. Mississippi saw its daily count of new cases reach new highs twice this week.
“It’s not a joke. Really bad things are going to happen,” Mississippi Health Officer Dr Thomas Dobbs said.
Republican Governor Greg Abbott of Texas, whose state was among the first to reopen, has put any further lifting of restrictions on hold.
He has also reimposed a ban on elective surgeries in some places to preserve hospital space after the number of patients statewide more than doubled in two weeks.
“The last thing we want to do as a state is go backwards and close down businesses,” Mr Abbott said.
The US recorded 34,500 COVID-19 cases Wednesday, slightly fewer than the day before but still near the high of 36,400 reached on April 24.
The daily average has climbed by more than 50 per cent over the past two weeks.
Deaths per day in the US are around 600 after peaking at about 2200 in mid-April.
“It is possible, if we play our cards badly and make a lot of mistakes, to get back to that level. But if we are smart, there’s no reason to get to 2200 deaths a day,” said Dr Ashish Jha, director of Harvard’s Global Health Institute.
Several states set single-day case records this week, including Arizona, California, Nevada, Texas and Oklahoma.
Mississippi’s Dobbs blamed a failure to wear masks and observe other social-distancing practices, and said many cases involved younger people spreading the virus to older relatives.
“I’m afraid it’s going to take some kind of catastrophe for people to pay attention,” he said.
“We are giving away those hard-fought gains for silly stuff.”
Tom Rohlk, a 62-year-old grocery store worker from Overland Park, Kansas, complained that young people sometimes act as if they don’t care: “It seems like it’s time to party.”
The US has greatly increased testing in the past few months, and it is now presumably finding many less-serious cases that would have gone undetected earlier in the outbreak, when the availability of testing was limited and sicker people were often given priority.
But there are other more clear-cut warning signs, including a rising number of deaths per day in states such as Arizona and Alabama.
More than 9.5 million people around the world have been diagnosed with COVID-19, while nearly 4.8 million have recovered, and more than 484,000 have died, according to Johns Hopkins University.
There have been more than 122,000 in the US, the world’s highest toll.
World Health Organization chief Tedros Adhanom Ghebreyesus said that while the COVID-19 pandemic is subsiding in Europe it is worsening elsewhere, with the number of infections expected to reach 10 million next week and the number of deaths 500,000.
“Globally, it’s still getting worse,” Dr Tedros said.