Democratic presidential nominee Joe Biden has criticised US President Donald Trump for downplaying the severity of coronavirus, saying there is “a lot to be concerned about”.
He said the president should be communicating the “right lesson” on masks and social distancing.
It came after Mr Trump told Americans not to fear Covid-19.
He has returned to the White House to continue his treatment for coronavirus after a three-night hospital stay.
The president, who is still contagious, removed his mask on the balcony of the White House while posing for pictures.
While he is no longer in hospital, his doctor has said he “may not entirely be out of the woods yet”.
Speaking at an NBC television town hall event in Miami, Florida, on Monday night, Mr Biden said he was “glad” that the president seemed to be recovering well.
But, he said, “I would hope that the president, having gone through what he went through… would communicate the right lesson to the American people. Masks matter.”
“The only thing I heard was one of the tweets saying that, you know, don’t be so concerned about all this, essentially.
“There’s a lot to be concerned about. Two hundred and ten thousand people have died,” he added.
The US has recorded more coronavirus cases and deaths than any other country in the world.
Former US First Lady Michelle Obama also criticised the president’s response to the virus in a video released on Tuesday in which she urged people to vote for Mr Biden “like your lives depend on it”.
“Seven months later, he [Mr Trump] still doesn’t have a plan for this virus. Seven months later, he still won’t wear a mask consistently and encourage others to do the same – even when those simple actions could save countless lives. Instead, he continues to gaslight the American people by acting like this pandemic is not a real threat,” she said.
Mr Trump’s diagnosis has upended his campaign for a second term in office, less than a month before the 3 November presidential election.
What happened during Trump’s return?
Wearing a navy business suit, tie and mask, Mr Trump walked out of Walter Reed National Military Medical Center in the Washington DC suburbs on Monday evening pumping his fist.
After a short helicopter ride, Mr Trump was pictured alone on the Truman Balcony of the White House. He removed his protective face mask, before giving a thumbs-up and a military-style salute.
A couple of hours later, he tweeted a campaign-style clip of his return set to stirring music.
Mr Trump also recorded a video message, urging Americans to get back to work.
“You’re going to beat it [coronavirus],” he told them, adding: “We’re going to be out front. As your leader, I had to do that. I knew there’s danger to it, but I had to do it. I stood out front, and led.”
Mr Trump also speculated: “Now I’m better, maybe I’m immune, I don’t know”.
The World Health Organization says it is too early to know if people who have recovered from Covid-19 are protected from a second infection, and if so, how long this protection might last. The president’s own medical team does not consider him to be fully recovered yet.
Mr Trump also promised that vaccines were “coming momentarily”, although the head of the US Centers for Disease Control said last month that he did not expect a vaccine to be generally available before the middle of next year.
Before leaving hospital, the president told Americans in a tweet not to fear the disease and said he would be back on the campaign trail “soon”.
According to US public health guidelines, Mr Trump should remain in isolation for up to 10 days after symptoms first appear. The White House says the president first started to appear ill on Thursday evening, and later tested positive.