Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, testifies before the U.S. Senate Committee on Health, Education, Labor and Pensions on COVID-19: Update on Progress Toward Safely Getting Back to Work and Back to School in Washington, D.C., the United States, on June 30, 2020. (Al Drago/Pool via Xinhua)

US top infectious diseases expert Anthony Fauci said on Monday the United States is seeing a surge in new COVID-19 infections because the country never shut down entirely, according to a CNBC report.

"We did not shut down entirely," said Fauci, director of the US National Institute of Allergy and Infectious Diseases, during a webinar with the Stanford School of Medicine.

"We need to draw back a few yards and say, 'OK, we can't stay shut down forever.' ...You've got to shut down but then you've got to gradually open," Fauci said.

Fauci said the United States has not "even begun to see the end" of the coronavirus pandemic yet as scientists continue to work on potential drugs and vaccines for the virus, according to the report.

He said he is "cautiously optimistic" scientists will be able to create at least one safe and effective vaccine by the end of the year or early 2021.

More than 3,353,000 COVID-19 cases have been reported in the United States with the fatalities surpassing 135,500 as of Monday evening, according to a tally by Johns Hopkins University.