A panel of health officials, including Dr Anthony Fauci, said the next few days will be crucial to stem the new outbreaks.
Cases are climbing rapidly across a number of US states.
The four top experts also testified they were never told by President Donald Trump to "slow down" testing.
Their comments come after Mr Trump told a weekend rally in Oklahoma that he had asked his team to do less testing to help keep official case counts down.
The White House has said his comment was "in jest".