US President Donald Trump on Tuesday denied unfounded rumors that a series of mini-strokes were behind his unscheduled visit to the Walter Reed Medical Center last year, calling the claims "fake news."

Trump made the surprise visit to the hospital in November. The fact that it was not scheduled, a departure from established protocols, ignited speculation at the time of a serious health issue, but the White House insisted he was just doing parts of his annual health exam ahead of time.

However CNN reported Tuesday that a new book by a New York Times journalist claims Vice President Mike Pence was put "on standby to take over the powers of the presidency temporarily" if Trump had to undergo anaesthesia during the visit.

CNN, which obtained a copy of the as-yet-unreleased book by Michael Schmidt, said the author did not specify the sourcing for the claim.

If true, it raises fresh questions about the visit.

The report quickly ignited claims on social media -- made without any evidence -- that Trump had suffered a stroke and had to be rushed to hospital, prompting the president to take to Twitter Tuesday.

"It never ends! Now they are trying to say that your favorite President, me, went to Walter Reed Medical Center, having suffered a series of mini-strokes. Never happened to THIS candidate - FAKE NEWS," Trump tweeted.

In what appeared to be a dig at the man challenging him for the presidency in November, Joe Biden, he added: "Perhaps they are referring to another candidate from another Party!"

Trump's personal physician Sean Conley insisted last year that the visit was nothing serious.

"Specifically, he did not undergo any specialized cardiac or neurologic evaluations," Conley said.