The White House's top infectious disease official said the number of Americans who could get sick or die from the coronavirus if the country does not implement an "aggressive" response could be "many, many millions".

Anthony Fauci, National Institute of Allergy and Infectious Diseases director, told the House Oversight and Reform Committee that Trump administration officials "cannot predict" a number of Americans who will contract the novel virus because it is still crafting and putting in place its response.

He warned lawmakers against "sitting back" and getting "complacent."

"It is going to be totally dependent on how we respond to it. I can't give you a realistic number" at this point, Mr Fauci told lawmakers.

But, if the federal response is not accurate, it "could be many, many millions," he said before quickly stressing that an "aggressive" response could allow federal, state and local governments "contain it."

His remarks were the latest contradictory ones from a Trump administration official. This time, Mr Fauci contradicted himself. All week, he has said on television interviews that federal officials have most past a containment-based strategy to one focused on "mitigation."

"We do not know what this virus will do," the top infectious disease official said before echoing Donald Trump by saying scientists typically would assume such a virus would dissipate as temperatures warm. But he said officials and experts know too little about the coronavirus to make that conclusion right now.

"We have to assume, it's going to get worse and worse and worse," Mr Fauci. "I can say we will see more cases and things will get worse than they are right now."

Trump administration officials have yet to clearly describe just what that means, and how that might affect the lives of those in the United States.