Leading US infectious disease expert Anthony Fauci on Tuesday warned Congress that lifting lockdowns too early could lead to more outbreaks of the coronavirus, which has already killed 82,000 in the country.
Mr Fauci, director of the National Institute of Allergy and Infectious Diseases, told a Senate panel that states should follow health experts' recommendations to wait for signs, including a declining number of new infections, before reopening.
President Donald Trump has been encouraging states to end a weeks-long shutdown of their economies.
But senators heard a sobering assessment from Mr Fauci when asked by Democrats about prematurely reopening the economy.
"There is a real risk that you will trigger an outbreak that you may not be able to control and, in fact, will set you back," he said.
Mr Fauci said that would lead to more suffering and death that could be avoided, and put the economy at even greater risk .
The Covid-19 respiratory disease caused by the coronavirus has infected almost 1.4 million people in the country, the most in the world.
Mr Fauci, 79, a member of Mr Trump's coronavirus task force, told the Senate health, education, labour and pensions committee that the battle against Covid-19 should be "focused on the proven public health practices of containment and mitigation".
He testified online from quarantine because he might have come into contact with two members of the White House staff who were diagnosed with Covid-19.
Mr Fauci said he might go to the White House if needed.
Last week, he was blocked by the White House from testifying to a Democratic-controlled House of Representatives panel.
The White House said Mr Fauci's testimony would have been "counterproductive".
"All roads back to work and back to school run through testing, and what our country has done so far on testing is impressive but not nearly enough," Lamar Alexander, the Republican chairman of the Senate committee, said by video on Tuesday.
Mr Alexander is also in quarantine in his home state of Tennessee for 14 days after a member of his staff tested positive.
Democrats on the health committee largely concentrated on the risks of opening the US economy too soon.
Republicans played down that notion, saying a long shutdown could cause serious harm to people's health and that of the economy.
Mr Trump, who previously made the strength of the economy central to his pitch for his November re-election, has encouraged states to reopen businesses that had been considered non-essential during the pandemic.
His administration has largely left it to states to decide whether and how to reopen.
State governors are taking varying approaches, with more relaxing tough restrictions, even as opinion polls show most Americans are concerned about reopening too soon.
Patty Murray, a senator and the senior committee Democrat, criticised aspects of the administration's response to the pandemic.
Ms Murray said Americans "need leadership, they need a plan, they need honesty and they need it now, before we reopen".
Others testifying online on Tuesday were: Robert Redfield, director of the US Centres for Disease Control and Prevention; Brett Giroir, assistant secretary for health; and Food and Drug Administration Commissioner Stephen Hahn.
Meanwhile, House majority leader Steny Hoyer, a Democrat, said a Democratic bill for significant new federal aid in response to the pandemic could be unveiled later on Tuesday, with a possible House votes on Friday.
