Dr Anthony Fauci said he was frustrated by the slump in vaccine uptake
The US is heading "in the wrong direction" on the coronavirus pandemic as infections surge among the unvaccinated, the country's top infectious disease expert has warned.
Dr Anthony Fauci said the Delta variant of Covid-19 was driving the spike in areas with low vaccination rates.
He said health officials were considering revising mask guidance for vaccinated Americans to curb cases.
Offering booster jabs to vulnerable people was also under review, he said.
The coronavirus situation in the US is becoming "a pandemic among the unvaccinated", Dr Fauci, the chief medical adviser to US President Joe Biden, told broadcaster CNN on Sunday.
More than 162.7 million people - or 49% of the population - have been fully vaccinated in the US, official data shows.
The US had been a world leader in jab uptake until April, when vaccination rates started to drop off.
Vaccination rates are particularly low in southern states, where fewer than half of residents have received their first dose in some cases.
Meanwhile, daily coronavirus infections are on the rise again after numbers fell in May and June.
The resurgence has added to the more than 34 million cases and 610,000 deaths recorded so far in the US.
The trend has been attributed, in part, to the rapid spread of the highly transmissible Delta variant of Covid.
The sharpest increases in Covid cases are in states with lower vaccination rates, such as Florida, Texas and Missouri.
Earlier this week, US Surgeon General Vivek Murthy said 99.5% of Covid deaths were occurring among unvaccinated people.
In his interview with CNN, Dr Fauci said local leaders in areas with low vaccination rates needed to do more to encourage people to get jabbed.
He said he was "very heartened to hear" the Republican governors of Arkansas and Florida promoting vaccinations in their states. Both leaders have been critical of Dr Fauci's advice in the past.
Dr Fauci has been a key adviser to President Biden and his predecessor, Donald Trump, through the pandemic.
He said he was involved in discussions to change mask guidance issued by the US Centers for Disease Control and Prevention (CDC).
As it stands, the CDC says people who are fully vaccinated can "resume activities that you did before the pandemic without wearing a mask".
Dr Fauci said local governments could issue their own rules under the current CDC guidance.
But he noted that some local governments had reinstated indoor and public mask requirements to curb rising infections.
In Los Angeles County, masks are now required indoors for both vaccinated and unvaccinated people.