Donald Trump gestures after landing at San Diego International Airport in San Diego, California, on Thursday. Nicholas Kamm / AFP

The US intelligence watchdog briefed lawmakers on Thursday about the handling of a whistleblower complaint on alleged behaviour by President Donald Trump, and a senior Democrat expressed alarm that the administration refuses to share the complaint with Congress.

The allegations, rejected by Trump as "presidential harassment," have set lawmakers on a collision course with the office of the Director of National Intelligence (DNI), which is refusing to share the details − raising suspicions the top spy official might be improperly protecting the president.

According to a report by The Washington Post, which cited two unnamed former US officials for its story, a complaint filed by a US intelligence official stemmed from Trump's communications with a foreign leader and a "promise" allegedly made by the president.

The foreign leader involved was not identified by the Post.

The newspaper did say the complaint "centers" on Ukraine, and cited two people familiar with the matter.

The New York Times also reported on Thursday that "at least part of" the whistleblower's complaint "deals with" Ukraine.

The newspaper cited two people familiar with the situation.

Trump dismissed the concerns in a Twitter thread, calling the story "fake news," and noting that any time he speaks with foreign leaders by phone he is well aware that there are likely "many people" listening from US agencies.

"Knowing all of this, is anybody dumb enough to believe that I would say something inappropriate with a foreign leader while on such a potentially 'heavily populated' call," he said.

"I would only do what is right anyway, and only do good for the USA!"

Trump spoke with Ukrainian President Volodymyr Zelensky, who was elected in May, about two weeks before the complaint was filed, according to both newspapers.

The whistleblower had filed a complaint with the inspector general of the intelligence community (IC IG), Michael Atkinson, who acknowledged that he considered it a credible matter of "urgent concern" that necessitated notifying congressional oversight committees.

But during a three-hour closed-door briefing, Atkinson told the House Intelligence Committee that the acting DNI, Joseph Maguire, has barred him from providing details of the complaint to Congress.

"That whole purpose is being frustrated here because the Director of National Intelligence has made the unprecedented decision not to share the complaint with Congress," the committee's chairman, Democrat Adam Schiff, told reporters after the briefing.

Schiff said, without revealing the source of his information, that the Department of Justice has been involved in the decision to withhold the whistleblower's complaint.

"We do not know, because we cannot get an answer to the question, about whether the White House is also involved in preventing this information from coming to Congress," he said.

Schiff made public on Thursday a letter from Atkinson in which the intelligence inspector general said that he and the acting DNI "are at an impasse over this issue."

Agence France-Presse