He made the remarks on Sunday during a TV programme while responding to a question as to whether he believes the true death toll is as high as suggested by a new study from the University of Washington, reports Xinhua news agency.
"You know, the model says that it's a significant amount, as you mentioned correctly, 900,000. That's a bit more than I would have thought the undercounting was, but, you know, sometimes the models are right on line, sometimes they're a bit off," he said.
"But I think there's no doubt that we are and have been undercounting. What that tells us is something that we've known.
"You know, we're living through a historic pandemic, the likes of which we haven't seen in over a hundred years," Fauci added.
As of Monday, the US' coronavirus death toll has increased to 581,752, according to the Johns Hopkins University.
The country's caseload stood at 32,707,359, also the largest in the world.