Top government scientist Anthony Fauci said on Sunday that an advertisement aired by Donald Trump's campaign was edited to make him appear to endorse the president's handling of the coronavirus pandemic.
"In my nearly five decades of public service, I have never publicly endorsed any political candidate," said Dr Fauci, the longtime director of the National Institute of Allergies and Infectious Diseases.
The 30-second campaign ad uses Mr Trump's personal experience with the virus.
"President Trump is recovering from the coronavirus and so is America," it says.
It includes a brief clip in which Dr Fauci, 79, appears to praise the president's response to the pandemic.
"I can't imagine that anybody could be doing more," he says.
But in the complete clip of Dr Fauci's comments in March to Fox News, he says: "I have been devoting almost full time on this. I'm down at the White House virtually every day with the task force.
"It's every single day. So I can't imagine that under any circumstances that anybody could be doing more."
On Sunday, Dr Fauci said: "The comments attributed to me without my permission in the GOP campaign ad were taken out of context from a broad statement I made months ago about the efforts of federal public health officials."
As a leader of the White House task force on the coronavirus, he has often had to walk a fine line in trying to clarify or correct the president's assertions about the disease or treatments and vaccines being developed against it.
Dr Fauci has at times aroused Mr Trump's anger, such as when the president in April retweeted a message containing the hashtag #FireFauci, before publicly insisting he was doing a great job.