Anthony Fauci said the number of cases had never reached a satisfactory baseline before the current resurgence, which officials have warned risks overwhelming hospitals in the country's south and west.
"It's a serious situation that we have to address immediately," Fauci said in a web interview with National Institutes of Health director Francis Collins.
But Fauci added he did not strictly consider the ongoing rise in cases a "wave."
"It was a surge or a resurgence of infections superimposed upon a baseline," he said, AFP reported.
"If you look at the graphs from Europe, the European Union as an entity, it went up and then came down to the baseline. Now they're having little blips, as you might expect, as they try to reopen. We went up, never came down to baseline, and now we're surging back up."
The death toll from the virus in the US hit 130,000 Monday, according to a tally by Johns Hopkins University, and the number of infections is nearing three million.
A worrying number of new cases were reported amid a resurgence that has forced several states to suspend phased economic reopening.
Fauci, who heads America's National Institute of Allergy and Infectious Diseases, is a leading member of President Donald Trump's coronavirus task force and has become a trusted face in the administration's battle against the epidemic.
The US is the world's hardest-hit nation from the virus and has been struggling to come to grips with a new normal of social distancing and mask-wearing.
Officials have warned that some of the country's hospitals are in danger of being overwhelmed by the influx of COVID-19 patients.
Hospital beds are full in parts of Texas, while calls for fresh stay-at-home orders are growing.
Some mayors have said their cities reopened too early as Trump tries to downplay the severity of the crisis, prioritizing economic reopening instead.