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.