U.S. President Joe Biden is not being treated for Parkinson's disease and has not seen a neurologist outside of his annual physicals, the White House said, after the New York Times reported that visitor logs showed a doctor specializing in the illness visited the White House at least eight times from August through March.

Concerns that Biden might be suffering from an undisclosed illness have risen since he stumbled, appearing frail and losing his train of thought at times, at his June 27 debate against Republican Donald Trump.

Dr. Kevin O'Connor, the White House doctor, issued a letter on Monday night that said Biden has not seen a neurologist outside of his normal annual physical.

Biden is battling criticism from some Democrats that he lacks the mental acuity to stand as their nominee against Trump in the Nov. 5 presidential election. Biden has, however, said he will not abandon his campaign, telling MSNBC via phone on Monday that he was "not going anywhere".

A Reuters review of White House visitor logs showed that Dr. Kevin Cannard, a neurologist and movement disorders specialist from Walter Reed National Military Medical Center, visited the White House eight times from August through March. Cannard has co-authored research on treatment for early Parkinson's disease at Vanderbilt University Medical Center.

O'Connor said Cannard's visits to the White House were part of his neurological clinics aimed at supporting "thousands" of active duty members assigned in support of White House operations and not to treat the president.

Cannard's visits to the White House were first reported by The New York Post on July 6.

One former White House official, who worked for Biden when he was vice president under President Barack Obama and as president, was treated by the same doctor for migraines in 2016, the former official told Reuters.

The doctor came to the White House once a month then as part of his rotation, the individual said.

White House spokesperson Karine Jean-Pierre refused to confirm or elaborate on Cannard's visits earlier in the day. At the media briefing, she was frequently challenged by reporters and said she wanted to respect the privacy of all involved for security reasons.

She said Biden had seen a neurologist three times connected to his annual physical exam. She did not explain Cannard's presence at the White House but suggested it might be connected to treating some of the military personnel who work at the White House complex.

"There are thousands of military personnel that come to the White House and they are under the care of the medical unit," she said.