The United States is heading "in the wrong direction" as the country scrambles to fight a new surge of COVID-19 cases because of the Delta variant and a large number of unvaccinated people, Anthony Fauci, White House chief medical advisor, has warned.

"We are going in the wrong direction," the top infectious disease expert told CNN on Sunday.

According to Fauci, the latest surge in new cases was caused by the highly virulent Delta variant, as well as the large unvaccinated population of the nation.

"It is really a pandemic among the unvaccinated, so this is an issue predominantly among the unvaccinated, which is the reason why we are out there, practically pleading with the unvaccinated people to go out and get vaccinated," said Fauci.

While the United States still has the highest number of confirmed cases, India had surpassed America in terms of new daily cases in the past few weeks. However, as infections started to stagnate in India, the United States again surpassed India's tally of new cases.

According to official data, the United States on Monday reported a staggering seven-day average of 52,000 infections. The figures have pushed America's tally of new daily infections past India's, according to Yahoo News.

A new study also found that the U.S. total number of COVID-19 infections may have been undercounted by as much as 60 percent.

According to a study by researchers at the University of Washington, as many as 65 million Americans may have already been infected.

As of Tuesday, the total confirmed cases of the pandemic surpassed 34.6 million, while the country's death toll amounted to over 611,410, according to data from Johns Hopkins University.

Last week, 48 states have a seven-day average of new cases at least 10 percent higher than the week before. In 34 of those states, the rate of new cases increased by more than 50 percent, said the university.

Meanwhile, Fauci has recently defended the natural origin theory of the novel coronavirus on several occasions.

On July 17, Fauci told CNN that "the most likely explanation is a natural evolution from an animal reservoir to a human."

He had also repeatedly cited a scientific paper that largely dismissed the possibility that the coronavirus escaped from a lab, according to media reports.

According to the weekly magazine Washington Examiner, in the paper, titled "The Origins of SARS-CoV-2: A Critical Review," a group of 20 internationally renowned virologists and evolutionary biologists from all over the world note that theories about a lab leak are almost all based on coincidence, not hard evidence.

Fauci was recently found to express his belief in natural origin theory of the novel coronavirus in his exchanges of emails with British zoologist Peter Daszak.

According to reports by Buzzfeed, Daszak thanked Fauci in an email on April 18, 2020, for "publicly standing up and stating that the scientific evidence supports a natural origin for COVID-19 from a bat-to-human spillover."

"From my perspective, your comments are brave, and coming from your trusted voice, will help dispel the myths being spun around the virus's origins," Daszak wrote then.