New Covid-19 cases in the US have risen to their highest level in two months, according to Johns Hopkins University, which is tracking the outbreak.
On Tuesday the US reported 34,700 new cases – the third highest daily tally since the US outbreak began, according to AP news agency.
Some southern and western states have been reporting record numbers of cases.
On Wednesday New York, New Jersey and Connecticut said they would quarantine people coming from hard-hit states.
Currently, those states are Alabama, Arkansas, Arizona, Florida, North Carolina, South Carolina, Texas, Washington and Utah, the governor of New York state Andrew Cuomo told reporters.
“This is a smart thing to do,” New Jersey’s Governor Phil Murphy said.
“We have taken our people, the three of us from these three states, through hell and back, and the last thing we need to do right now is subject our folks to another round.”
Health officials say the coming weeks will be crucial to stem the outbreaks.
On Tuesday America’s top infectious disease expert Dr Anthony Fauci told lawmakers there was “a disturbing surge of infection” and “increased community spread” in many southern and western states.
“A couple of days ago there were 30,000 new infections” in a single day, he said. “That’s very troubling to me.”
He told Americans to take social distancing measures, saying: “Plan A, don’t go in a crowd. Plan B, if you do, make sure you wear a mask.”
The US has recorded more than 2.3 million cases of the virus and more than 120,000 deaths.
The overall number of infections in the US rose by a quarter last week, with 10 states reporting a rise in cases of more than 50%, according to Reuters news agency.
On Wednesday Florida reported a record 5,508 new infections, media in the state said quoting local health officials. It brings Florida’s total number of confirmed infections to 109,014, with 3,281 deaths.
In the absence of a state-wide mask requirement, several cities and towns have issued their own orders mandating masks in public.
Arizona on Wednesday reported 1,795 new infections, down from the record 3,600 new infections on Tuesday, when President Trump held a campaign rally in the city of Phoenix.
Phoenix’s Democratic Mayor Kate Gallego said Mr Trump’s event would not be safe and urged him to wear a mask.
But the president, who has so far refused to wear a mask, insisted at the event that the coronavirus “plague” was “going away” and again referred to the virus as the “kung flu”, which the White House denies is a racist term.
Arizona officials warn that over 80% of hospital beds are currently being used, and that the healthcare system may be overrun in the coming days or weeks.
In Texas, where record numbers of people have been admitted to hospital for 12 days in a row, a children’s hospital in Houston has begun admitting adult virus patients.
Texas Children’s Hospital urged the public to “take responsible actions – practice appropriate social distancing, wear a mask or face covering anytime you leave your home”.
Both Arizona and Texas were among those states that removed coronavirus restrictions early. Texas has temporarily revoked alcohol licences from several businesses that were breaking social distancing rules.
On Tuesday the governor of Washington state, Jay Inslee, ordered all residents to wear face masks in public after confirmed infections rose by more than a third the previous week.
“This is about saving lives. It’s about reopening our businesses. And it’s about showing respect and care for one another,” Mr Inslee said.
Several US states and cities have instructed residents to wear face coverings.
California, South Carolina, Utah, Mississippi and Louisiana have also seen a surge in caseload and some governors have said they may be forced to announce new lockdown measures.
The European Union is reportedly considering banning US citizens from entering the bloc as it considers how to reopen its external borders.
Dr Fauci and other health experts told Congress on Tuesday that they were never advised by President Donald Trump to “slow down” testing and said they planned to increase testing.
Their comments come after Mr Trump told a weekend rally in Oklahoma that he had asked his team to do less testing to help keep official case counts down.
The White House has said the president’s comment was “in jest”. But on Tuesday the president appeared to contradict that, telling reporters: “I don’t kid.”
Brett Giroir, the health department assistant secretary who oversees US diagnostic capacity, told lawmakers he expected the US would be able to conduct 40 to 50 million tests per month by autumn.
CDC Director Dr Robert Redfield called testing “a critical underpinning of our response”, but said social distancing measures were more effective at keeping the virus from spreading.
Dr Redfield also called on all Americans to get a flu jab this year, saying the public must “embrace flu vaccinations with confidence”.
“This single act will save lives,” he added.
Dr Fauci added that he was “cautiously optimistic” a vaccine might be ready by the end of 2020. He said it was a matter of “when and not if” the vaccine is ready, but added that it “might take some time”.
Drug company Moderna has plans “to launch a Phase 3 clinical trial as early as July 2020, pending positive results from this Phase 2 trial”, he said.
Dr Fauci also defended his decision not to warn Americans to start wearing masks earlier, saying it was due to a scarcity at the time of personal protective equipment, or PPE, which was needed for healthcare providers.