The director of the Centres for Disease Control and Prevention said on Wednesday that face masks are more guaranteed to protect from coronavirus than a vaccine.
It comes a day after Donald Trump questioned the effectiveness of masks during ABC News' town hall in Pennsylvania on Tuesday night.
Speaking at a Senate hearing on the federal coronavirus response, CDC director Robert Redfield said if all Americans embraced masks for six to 12 weeks, they would bring the pandemic under control.
"I'm not going to comment directly about the president but I am going to comment as the CDC director that face masks, these face masks are the most important powerful public health tool we have," Mr Redfield said.
"These actually, we have clear scientific evidence they work, and they are our best defence. I might even go far as to say this face mask is more guaranteed to protect me against Covid, than when I take a Covid vaccine because the Immunogenicity may be 70 per cent, and if I don't get an immune response the vaccine's not going to protect me."
A day earlier, Mr Trump told ABC's George Stephanopoulos that "a lot of people" questioned the effectiveness of wearing masks when discussing the coronavirus response.
"A lot of people got it wrong. They talked about ‘don’t wear masks’, and now they say ‘wear masks’. Although some people say don’t wear masks," Mr Trump said.
When Mr Stephanopoulos asked which people don't think masks are a good idea, the president pointed to waiters and, in previous statements, Dr Anthony Fauci and the CDC.
"They come over and they serve you, and they have a mask. And I saw it the other day where they were serving me, and they're playing with the mask ... I’m not blaming them ... I’m just saying what happens. They're playing with the mask, so the mask is over, and they're touching it, and then they're touching the plate. That can’t be good," he said.
"If you look at Dr Fauci’s original statement ... you look at a lot of people, CDC, you look at a lot of people’s original statement, they said very strongly, George, don’t wear masks. Then all of a sudden they went to wear masks.
"The concept of a mask is good, but it also does ... you’re constantly touching it, you’re touching your face, you’re touching plates. There are people that don’t think masks are good."
Mr Redfield, however, said at the hearing on Wednesday that he appealed for all Americans to take personal responsibility and embrace the face coverings.
"So I do want to ask the American public to take the responsibility, particularly the 18 to 25-year-olds where we're seeing the outbreak in America continue to go [up]," he said.