The United States added 792 deaths in 24 hours, the Johns Hopkins University tally showed Tuesday, as the country's top infectious diseases expert warned there were "historic" challenges ahead.

In this file photo taken on April 30, 2020 the hospital ship USNS Comfort passes the Statue of Liberty as its departs New York City. (File photo by AFP)

The world's largest economy is the hardest-hit country by the pandemic, with 121,176 dead out of more than 2.34 million cases, according to the Baltimore-based institution at 8:30pm (0030 GMT Wednesday).

Many states have largely lifted lockdown measures, and New York -- the country's epicenter for the pandemic -- took a big step Monday by allowing non-essential businesses to reopen.

But some 20 states, primarily in the south and west, have seen a rebound in infections.

Earlier Monday, US government health experts led by Anthony Fauci warned Congress that the country faces "historic" challenges with the virus and should prepare for a lengthy battle.

Fauci, who has become the trusted face of the country's pandemic response, added that the coming two weeks will be "critical to our ability to address... surgings" in states like Florida, Texas and Arizona.