America's top infectious disease expert has told lawmakers that the US is seeing a "disturbing surge" in coronavirus infections in some states.
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.
"To my knowledge, none of us have ever been told to slow down on testing," Dr Fauci, the director of the National Institute of Allergy and Infectious Diseases, testified to a congressional committee investigating the US response to the pandemic.
"In fact, we will be doing more testing," he added.
The other three officials - representing the Centers for Disease Control and Prevention (CDC), the Food and Drug Administration (FDA) and the Department of Health and Human Services - also disputed Mr Trump's comment, saying they had never been directed to slow testing.
Brett Giroir, the health department assistant secretary who oversees US diagnostic capacity, told lawmakers he expects the US will be able to conduct 40 to 50 million tests per month by autumn.
What did Trump say?
The White House has said the president's comment about slowing testing was "in jest". But on Tuesday the president appeared to contradict that, telling reporters: "I don't kid."
About 2.3 million Americans have been infected with coronavirus and at least 120,000 have died - more than any other nation.
But Mr Trump told a campaign event in Phoenix, Arizona, later in the day that the coronavirus "plague" was "going away".
The president once again referred to the virus as the "kung flu", which the White House denies is a racist term.
Arizona, where the president addressed a megachurch on Tuesday, surpassed its daily record for new infections only hours before he arrived.
Officials there warn that over 80% of hospital beds are currently being used, and that the healthcare system may be overrun in the coming days or weeks.
What else did experts say?
Dr Fauci warned of "a disturbing surge of infection" and "increased community spread" in many southern and western states.
"A couple of days ago there were 30,000 new infections" in a single day, he said. "That's very troubling to me."
"The next couple of weeks are going to be critical to address those surges that we're seeing in Texas, Florida, Arizona, and other states."
CDC Director Dr Robert Redfield called testing "a critical underpinning of our response", but said social distancing measures are more effective at keeping the virus from spreading.
Dr Redfield also called on all Americans to get a flu jab this year, saying the public must "embrace flu vaccinations with confidence".
"This single act will save lives," he added.
Dr Fauci added that he is "cautiously optimistic" a vaccine may be ready by the end of 2020.
He said it is a matter of "when and not if" the vaccine is ready, but added that it "might take some time".
Drug company Moderna has plans "to launch a Phase 3 clinical trial as early as July 2020, pending positive results from this Phase 2 trial", he said.
Dr Fauci also defended his decision not to warn Americans to start wearing masks earlier, saying it was due to a scarcity at the time of personal protective equipment, or PPE, which was needed for healthcare providers.
Several cities and counties have issued new mask requirements in the past week.
"Plan A: Don't go in a crowd. Plan B: If you do, make sure you wear a mask," Dr Fauci told the committee.
Where else is the virus surging?
The number of new daily infections is increasing in more than half of all US states.
Along with Arizona, Nevada and Texas have also just notched up single-day records for new coronavirus cases.
Other states, including California, South Carolina, Utah, Mississippi and Louisiana, have seen a surge in caseload.
Some governors have said they may be forced to announce new lockdown measures.
On Monday, Florida surpassed 100,000 cases. In the absence of a state-wide mask requirement, several cities and towns have issued their own orders mandating masks in public.
Texas has temporarily revoked alcohol licences from several businesses that were breaking social distancing rules.
The governor of Washington on Tuesday ordered residents to wear face masks in public after the north-western US state saw its positive tests rise by 35% last week.