A mutation of coronavirus able to spread more easily may have emerged, America's top infectious disease expert has warned, as Texas made mask-wearing in public mandatory.
Dr Anthony Fauci, who sits on the White House coronavirus task force, said there is data to suggest the existence of a new mutation of Covid-19 which is more "transmissible".
He cautioned that scientists are still trying to confirm its existence and it does not appear to be more damaging for patients. However, the potential is a cause for concern as the United States grapples with record highs of new daily coronavirus cases.
Texas, one of the states seeing a surge in case numbers and hospital admissions, ordered the use of face masks in certain situations.
Texans in counties with 20 or more Covid-19 cases - the vast majority of the state - must have their nose and mouth covered in outdoor public spaces or a building open to the public.
Greg Abbott, the Republican governor of Texas, said if the state wanted businesses to stay open, it was vital to stop the virus's spread.
The move comes as senior Republicans urge Americans to wear face masks in public.
Mitch McConnell, the top Republican in the Senate, and Steven Mnuchin, the Treasury secretary, issued public calls for people to do so in recent days.
Donald Trump also said he was "all for masks" and joked in a recent interview that he looked like the Lone Ranger when wearing one, adopting a less critical tone than in the past. Yet his plan last night to attend a July 4 celebration at Mount Rushmore in South Dakota came in for criticism.
The event, due to feature a fireworks display, is expected to be attended by 7500 people. Face masks are to be handed out, but attendees are not required to socially distance.
Kristi Noem, the state's Republican governor, told Fox News social distancing would not be needed at the event so people could "enjoy the freedoms and liberties in this country".
Fauci made his comments about a mutation of Covid-19 during an appearance on a podcast run by The Journal of the American Medical Association.
"I think the data is showing that there is a single mutation that actually makes the virus be able to replicate better and maybe have high viral loads," Fauci said.
"We don't have a connection to whether an individual does worse with this or not. It just seems that the virus replicates better and may be more transmissible."
Fauci, the director of the National Institute of Allergy and Infectious Diseases, said confirmation was still being sought. Viruses naturally mutate and scientists have previously found evidence of minor mutations of Covid-19.
The World Health Organisation is monitoring more than 60,000 different genetic sequences of coronavirus.
In America, 55,000 new cases of Covid-19 were confirmed on Thursday, about 20,000 more cases than the first peak in April.
Now, 36 of the 50 US states are seeing cases rise, according to a CNN tracker. The surge has put parts of the country that had been reopening into reverse.