Donald Trump and one of his top public health experts had some advice for young people – including millennials – by imploring them to avoid large groups and warning they could give their older loved ones coronavirus.

Health experts say those over 60 years are most vulnerable to the disease, noting most US deaths and those elsewhere have occurred to people that age and older. Those experts also warn children and young adults can carry COVID-19 while feeling no symptoms.

The president, during a Wednesday press briefing at the White House offered some rather measured advice for the Millennial generation.

"I hope they just listen to what we've been saying over the last period of time. We don't want them gathering, and I see that they do gather including on beaches, and including in restaurants, young people," he said.

"They don't realise that -- they're feeling invincible, I don't know if you felt invincible when you were young," he said to the reporter who asked a question about younger Americans. "But they don't realise that they could be carrying lots of bad things home to their grandmother and grandfather and even their parents."

Federal officials "want them to heed the advice," he said of Washington's guidelines that include avoiding bars and restaurants, "and I do believe it's getting through."

Mr Trump did not explain why he feels that way as a new NPR/PBS NewsHour/Marist poll released Tuesday showed he has some work to do among young people. That survey found just 36 per cent of Americans under 45 years old approve of his handling of the federal coronavirus response.

That number may not have been helped by more blunt comments one of his COVID-19 task force members, Deborah Birx, made towards that group.

"So again, I'm going to call on that generation, as part of that group that brought us innovation, particularly throughout all of their ability to look around corners and skip through [video] games on -- I always went level by level. I didn't realize that you could go from level three to level seven," she said. "That's what they taught us. They look for things that we don't see. We need them to be healthy."

She also revealed some statistics about millennials in other countries hit hard by the virus.

"It may have been that [within] the millennial generation ... there may be disproportional number of infections among that group and so even if it's a rare occurrence it may be seen more frequently in that group," Ms Birx said.

"There are concerning reports coming out of France and Italy about some young people getting seriously ill and very seriously ill in the ICUs," she added. "We think part of this may be that people heeded the early data coming out of China and coming out of South Korea of the elderly or those with pre-existing medical conditions were a particular risk."