Top US virus researcher Dr Anthony Fauci has called President Donald Trump's sharing of a video which included claims masks are not needed to fight Covid-19 "not helpful".
The video promotes a drug widely disproven to be effective in treating Covid-19.
Dr Fauci's interview with the BBC's Katty Kay comes as the US is about to hit 150,000 deaths due to the pandemic.
The virus continues to spread rapidly in the US as states lock down again.
President Trump was among social media users who shared video on social media late on Monday of a group called America's Frontline Doctors advocating hydroxychloroquine as a Covid-19 treatment and saying that masks and shutdowns are not effective in combating coronavirus.
Facebook and Twitter removed the video, flagging it as misinformation, but not before more than 17 million people had viewed it.
The speaker in the video also alleged that there was a conspiracy to prevent the world from learning of a coronavirus "cure" that was being led "by Fauci & the Democrats to perpetuate Covid deaths to hurt Trump". On Tuesday, Dr Fauci denied "misleading the American public under any circumstances".
What did Dr Fauci say?
"This issue of tweeting and retweeting is something that I've never understood or gotten involved in," Dr Fauci told BBC News on Wednesday.
"But I can tell you, you'd have to say it's not helpful if people get signals about not wearing masks when we are trying to get people to universally wear masks."
"My feeling about what we should do with masks is very very clearly understood by everyone including those in the White House," added Dr Fauci, who serves on the White House coronavirus task force and has directed the US National Institute of Allergy and Infectious Diseases since 1984.
Asked about Mr Trump's promotion of hydroxychloroquine – a drug Mr Trump himself says he took to ward off the virus – Dr Fauci said it would not be "productive or helpful for me to be making judges [sic] on right or wrong".
"We know that every single good study – and by good study I mean randomised control study in which the data are firm and believable – has shown that hydroxychloroquine is not effective in the treatment of Covid-19," he said.
Last month, the US Food and Drug Administration (FDA) cautioned against using the drug to treat coronavirus patients, following reports of "serious heart rhythm problems" and other health issues.
Studies commissioned by the WHO, the US National Institutes of Health and other researchers around the world have found no evidence that hydroxychloroquine – when used with or without the antibiotic azithromycin, as repeatedly recommended by President Trump – helps treat coronavirus.
A 26 July report from the White House coronavirus taskforce identified 21 states as being in the "red zone", meaning that they had reported more than 100 new cases per 100,000 residents in the past week.
Twenty-eight states are in the "yellow zone". Vermont is the only state in the green, with fewer than 10 cases per 100,000 people in the past week.
The report calls for further closures by state governors to contain the spread of the outbreak.
The video retweeted by Mr Trump showed doctors speaking outside the US Supreme Court building at an event organised by Tea Party Patriots Action, a group that has helped fund a pro-Trump political action committee.
In the video, Dr Stella Immanuel, a doctor from Houston, says she has successfully treated 350 coronavirus patients "and counting" with hydroxychloroquine.
The president said on Tuesday: "I think they're very respected doctors. There was a woman who was spectacular in her statements about it."