US House Speaker Nancy Pelosi has attacked the White House coronavirus task force's Deborah Birx, linking her to "disinformation" spread by President Donald Trump.
Dr Birx responded that she always based her decisions on scientific data.
She is a leading member of the task force, working alongside infectious diseases chief Anthony Fauci.
Trump has regularly drawn criticism for his claims about Covid-19, many of which have been proved to be false.
What did Pelosi say?
Ms Pelosi made the comments on ABC's This Week programme, in response to a question about a report on the Politico website that she had criticised Dr Birx in a meeting with other administration officials.
"I think the president has been spreading disinformation about the virus and she is his appointee, so I don't have confidence there, no," Ms Pelosi told ABC.
According to Politico, the House speaker used stronger words in her meeting last week with Treasury Secretary Steven Mnuchin and White House Chief of Staff Mark Meadows.
"Deborah Birx is the worst. Wow, what horrible hands you're in," she is quoted as saying to them. She also described Dr Fauci as a "hero".
Dr Birx told CNN's State of the Union she respected Ms Pelosi, and thought the criticism was triggered by a New York Times article which suggested she was too ready to embrace overly optimistic assessments about the pandemic.
"This was not a pollyannish view. I've never been called pollyannish, or non-scientific, or non-data driven," she said. "I will stake my 40-year career on those fundamental principles of using data to implement better programmes and save lives."
In the same interview, Dr Birx said the US had entered a "new phase" in its fight against the pandemic, with the disease being "extraordinarily widespread".
She cautioned people living in rural areas that they were not immune and must take precautions such as wearing face-coverings and social distancing.
She also urged Americans returning from holiday to assume that they were infected and to self-isolate - and she said people living in multi-generational households should consider wearing a mask while at home.
