The top US infectious diseases doctor has warned senators the virus will spread if the country opens up too soon.
Dr Anthony Fauci said if federal guidelines to reopen are not followed, “little spikes” will become outbreaks.
He also said the real US death toll is probably higher than the official figure of 80,000.
His message is at odds with the upbeat tone of President Trump who is keen to get the economy going again.
Dr Fauci was speaking via video to a Republican-led committee of the US Senate.
He was referring to the White House’s Opening Up America Again plan, which includes three 14-day phases that states are urged to consider implementing as they allow schools and businesses to reopen.
Several US states already restarting their economies have infection rates that are rising, not falling.
He warned of the risk of triggering an outbreak that officials would not be able to control, adding that such an outbreak would set back economic recovery and could lead to “suffering and death”.
Although the White House has laid out guidelines for reopening, it is ultimately up to state governors to make the decisions on how to ease the lockdown.
“There is no doubt, even under the best of circumstances, when you pull back on mitigation you will see some cases appear,” Dr Fauci warned the US officials.
Questioned about the possibility of a rebound in autumn of the virus, Dr Fauci said one is “entirely conceivable and possible”.
“I hope that if we do have the threat of a second wave we will be able to deal with it very effectively to prevent it becoming an outbreak.”
Dr Fauci also said there are multiple vaccines in development but “no guarantee” any will be effective, though based on his knowledge of other viruses, he is “cautiously optimistic”.
“We have many candidates and hope to have multiple winners,” he said. “In other words it’s multiple shots on goal.”
Witnesses testify remotely
Witnesses appeared remotely during the over three-hour long hearing, as did some senators on the Senate Committee on Health, Education Labor and Pensions.
The three members of the White House coronavirus task force who testified are self-isolating after possible exposure to the illness, including Dr Fauci.
Centers for Disease Control and Prevention director Dr Robert Redfield and Food and Drug Administration commissioner Stephen Hahn are also self-isolating.
Dr Fauci has tested negative but will continue to work from home for the time being, and will be regularly tested.
The hearing was led by Republican Tennessee Senator Lamar Alexander from his home in the Smoky Mountains. His dog Rufus was occasionally seen slumbering in the background of the video conference call.
“Staying at home indefinitely is not a solution to this pandemic,” said Mr Alexander, adding: “There is not enough money available to help all of those hurt by a closed economy.”
Washington Senator Patty Murray, the highest-ranking Democrat on the committee, charged that Mr Trump is more interested in “fighting against the truth than fighting the virus”.
The government cannot ask people to restart their lives “if there isn’t clear detailed guidance on how to do that safely”, she adds.
Some US states are beginning to lift lockdown orders. Republican-run states such as Georgia, Oklahoma and Alaska have moved first towards relaxing restrictions.
Tuesday’s hearing was Dr Fauci’s first appearance before lawmakers since President Donald Trump declared a state of national emergency in March.
The senior health adviser – who has become the public face of the fight against the virus in the US – was blocked from testifying to a Democratic-led House committee examining the Trump administration’s response to the pandemic on May 6.
What’s the situation like in the US?
The United States alone has more than 1.3 million confirmed cases, according to the Johns Hopkins University coronavirus tracker – almost six times as many as any other country.
President Trump claimed that the US had “prevailed” in testing people for coronavirus infections in a news conference on Monday.
But as of this week, the US has tested only 2.75% of its 330m population, and no state has tested 10% of residents.
In a separate development, White House staff have been ordered to wear masks when entering the West Wing after two aides tested positive for coronavirus.
Mr Trump said he did not need to follow the directive as he kept “far away from everyone”.