Top immunologist Anthony Fauci has cautioned against the premature opening of the US economy, warning that the "consequences could be really serious".
Appearing by teleconference at a highly-anticipated Senate committee hearing on Tuesday, Dr Fauci said states needed to follow the guidelines announced by the White House coronavirus task force last month before reopening.
If they failed to do so there was “a real risk that you will trigger an outbreak you might not be able to control”, he said, adding that this could lead to avoidable “suffering and death” and further economic damage.
“There is no doubt that even under the best circumstances when you pull back on mitigation you will see some cases appear. It is the ability and capability of responding to those cases with good identification and isolation techniques that determines whether you continue to go forward in trying to reopen America.”
Dr Fauci also said he believed the death toll from coronavirus in the United States is higher than has been reported.
"The number is likely higher – I don't know the exact per cent higher – but it almost certainly is higher," he said, in response to a question from Senator Bernie Sanders, who suggested that the total deaths from Covid-19 could be up to 50 per cent higher than the current confirmed level of 80,000.
Dr Fauci's comments appear to put him at odds with President Donald Trump who has claimed that "big progress" has been made in the fight against coronavirus and has encouraged states to reopen.
The 79-year-old immunologist, who has worked for six American presidents during his career as a top infectious disease expert, is a senior member of the task force and has been the face of the federal response to the corona virus outbreak to many Americans.
However, Mr Trump’s decision to end the daily televised task force briefing has meant that Dr Fauci has been less visible in recent weeks just as states have begun to lift restrictions.
During the hearing Dr Fauci tempered expectations about the potential for certain drugs to treat Covid-19.
Remdesivir, a drug that gained fast-track approval by the Food and Drug Administration this month, had delivered "only a modest result", boosting recovery time in patients by 31 per cent, he said.
He also played down hopes that a vaccine against the virus is imminent, predicting that it would be a “bridge too far” to suggest vaccines could be ready in time for the return of students to university in the autumn.
Instead, testing and contract tracing was the key to slowing the spread of the virus, he said.
Dr Fauci was one of four witnesses to appear remotely at Tuesday's hearing, while committee chair Lamar Alexander also presided over proceedings from his home in Tennessee after one of his staff members was found to have contracted the virus. Dr Fauci has self-isolated since at least two members of the White House staff tested positive for the virus last week.
The White House has barred Dr Fauci and other officials from appearing before committees in the House of Representatives which, unlike the Senate, is controlled by Democrats.
Opening the hearing, Mr Lamar noted that the US had improved testing levels, and had now completed more than nine million tests – twice as many as any other country, and the most per capita of many countries including South Korea.
He said everyone had underestimated the virus, how contagious it could be and how deadly it could be for some sectors of the population. “Even the experts underestimated Covid-19,” he said.
Senator Patty Murray, the top Democrat on the committee, criticised the Trump administration's response to the crisis as a "disaster" that had been beset by "delays, missteps". Truth is essential in combatting the virus she said. "If the president isn't telling the truth we must and our witnesses must."