Dr Deborah Birx said there were good signs the outbreak was stabilising, but cautioned: “As encouraging as they are, we have not reached the peak.”
President Donald Trump also said he expects the US to see a lower death toll than the initial predictions of 100,000 fatalities.
The US has over 475,000 confirmed cases and nearly 18,000 deaths so far.
Dr Birx said the rate of increase appears to be stabilising in hard-hit regions like New York, New Jersey and the city of Chicago.
But she emphasised the nation had yet to see the peak of the outbreak and mitigation efforts should be kept up.
“We need to continue to do what we did yesterday, and the week before, and the week before that because that’s what, in the end, is going to take us up across the peak and down the other side.”
The Institute for Health Metrics and Evaluation in Washington state has predicted the nation’s death count will peak on Friday and then begin declining.