WASHINGTON — The government's intelligence watchdog declined on Thursday to discuss the substance of a whistleblower complaint that reportedly concerns President Donald Trump making an unspecified promise to a foreign leader. Trump rejected the report as fake news and "presidential harassment."

The inspector general's testimony at a closed-door meeting of the House intelligence committee was described by two people with knowledge of the proceedings. They were not authorized to discuss the meeting by name and were granted anonymity.

The Washington Post reported the complaint involves an intelligence official's allegation that Trump made the promise to an unidentified foreign leader in a telephone call. The Post cited two anonymous former U.S. officials.

The Associated Press has not confirmed the report.

"Another Fake News story out there - It never ends!" Trump tweeted. "Virtually anytime I speak on the phone to a foreign leader, I understand that there may be many people listening from various U.S. agencies, not to mention those from the other country itself. No problem!"

He asked: "Is anybody dumb enough to believe that I would say something inappropriate with a foreign leader while on such a potentially "heavily populated" call."

Intelligence Committee Chairman Adam Schiff, D-Calif., said inspector general Michael Atkinson determined the whistleblower complaint was "credible and urgent" and should be "transmitted to Congress."

Atkinson testified behind closed doors at the Capitol, but it appeared he did not disclose details to lawmakers. Instead the appearance mainly involved a discussion of the process for whistleblower complaints, the sources said.

Joseph Maguire, acting director of national intelligence, has refused to discuss details.

Schiff subpoenaed Maguire, saying he was withholding a whistleblower complaint from Congress and questioning whether he had been directed to do so by the White House or the attorney general. Maguire is expected to testify publicly about the whistleblower complaint on Sept. 26.

Both Atkinson and Maguire are to appear next week before the Senate intelligence committee, according to Sen. Mark Warner of Virginia, the ranking Democrat on the panel.

Rep. Jim Himes, D-Conn., said Thursday on MSNBC that the acting director "broke the law when he decided to basically intercept the inspector general's report to Congress."

That's "never been done before in the history of inspector general reports to the Congress," Himes said. "And the American people should be worried about that."

Himes said ahead of the meeting that lawmakers are in the uncomfortable position of not knowing any more than what's in the news reports.

"We don't know exactly what is in the substance of this complaint," he said. "It could be nothing. It could be something very, very serious."

Schiff did not divulge the subject of the complaint, but said the committee "places the highest importance on the protection of whistleblowers and their complaints to Congress."

In a letter Tuesday, the general counsel for the Office of the Director of National Intelligence, Jason Klitenic, wrote that the agency is protecting the whistleblower and argued the allegation does not meet the definition of "urgent concern." He said the complaint "concerned conduct from someone outside the intelligence community and did not relate to 'intelligence activity' under the DNI's supervision."

Schiff said last week that Maguire is required to share the complaint with Congress and said the attempt to hold it back "raises serious concerns about whether White House, Department of Justice or other executive branch officials are trying to prevent a legitimate whistleblower complaint from reaching its intended recipient, the Congress, in order to cover up serious misconduct."

AP writer Zeke Miller contributed.