WASHINGTON - The Trump administration plunged into an extraordinary showdown with Congress over access to a whistleblower's complaint about reported incidents including a private conversation between President Donald Trump and a foreign leader. The blocked complaint is "serious" and "urgent," the government's intelligence watchdog said.

The administration is keeping Congress from even learning what exactly the whistleblower is alleging, but the intelligence community's inspector general said the matter involves the "most significant" responsibilities of intelligence leadership. A lawmaker said the complaint was "based on a series of events."

The Washington Post and The New York Times reported Thursday that at least part of the complaint involves Ukraine. The newspapers cited anonymous sources familiar with the matter. The Associated Press has not confirmed the reports.

The inspector general appeared before the House intelligence committee behind closed doors Thursday but declined, under administration orders, to reveal to members the substance of the complaint.

The standoff raises fresh questions about the extent to which Trump's allies are protecting the Republican president from oversight and, specifically, if his new acting director of national intelligence, Joseph Maguire, is working with the Justice Department to shield the president from the reach of Congress.

Trump, though giving no details about any incident, denied Thursday that he would ever "say something inappropriate" on such a call.

Rep. Adam Schiff, D-Calif., the chairman of the House Intelligence Committee, said he was prepared to go to court to try to force the Trump administration to open up about the complaint.

"The inspector general has said this cannot wait," said Schiff, describing the administration's blockade as an unprecedented departure from law. "There's an urgency here that I think the courts will recognize."

Schiff said he, too, could not confirm whether newspaper reports were accurate because the administration was claiming executive privilege in withholding the complaint. But letters from the inspector general to the committee released Thursday said it was an "urgent" matter of "serious or flagrant abuse" that must be shared with lawmakers.

The letters also made it clear that Maguire consulted with the Justice Department in deciding not to transmit the complaint to Congress in a further departure from standard procedure. It's unclear whether the White House was also involved, Schiff said.

Because the administration is claiming the information is privileged, Schiff said he believes the whistleblower's complaint "likely involves the president or people around him."

Trump dismissed it all.

"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."

House Democrats are fighting the administration separately for access to witnesses and documents in impeachment probes. Democrats are also looking into whether Trump lawyer Rudy Giuliani traveled to Ukraine to pressure the government to aid the president's reelection effort by investigating the activities of potential rival Joe Biden's son Hunter, who worked for a Ukrainian gas company.

During an interview Thursday on CNN, Giuliani was asked whether he had asked Ukraine to look into Biden. Giuliani initially said, "No, actually I didn't," but seconds later he said, "Of course I did."

Later, Giuliani tweeted, "A President telling a Pres-elect of a well known corrupt country he better investigate corruption that affects US is doing his job."

Among the materials Democrats have sought in that investigation is the transcript of a phone call Trump had with Ukrainian President Volodymyr Zelenskyj on July 25.

This new situation, stemming from the whistleblower's Aug. 12 complaint, has led to their public concerns that government intelligence agencies and the recently named acting director might be under pressure to withhold information from Congress.

Trump tapped Maguire, a former Navy official, as acting intelligence director in August, after the departure of Director Dan Coats, a former Republican senator who often clashed with the president, and the retirement of Sue Gordon, a career professional in the No. 2 position.

Maguire has refused to discuss details of the whistleblower complaint, but he has been subpoenaed by the House panel and is expected to testify publicly Sept. 26. Maguire and the inspector general, Michael Atkinson, also are expected next week at the Senate intelligence committee.

Atkinson wrote in letters that Schiff released Thursday that he and Maguire had hit an "impasse" over the acting director's decision not to share the complaint with Congress.

While Atkinson wrote that he believed Maguire's position was in "good faith" it did not appear to be consistent with past practice. Atkinson said he was told by the legal counsel for the intelligence director that the complaint did not actually meet the definition of an "urgent concern." And he said the Justice Department said it did not fall under the director's jurisdiction because it did not involve an intelligence professional.

Atkinson said he disagreed with that Justice Department view. The complaint "not only falls under DNI's jurisdiction," Atkinson wrote, "but relates to one of the most significant and important of DNI's responsibilities to the American people."

The inspector general went on to say he requested authorization to at the very least disclose the "general subject matter" to the committee but had not been allowed to do so. He said the information was "being kept" from Congress. These decisions, the inspector general said, are affecting his execution of his duties and responsibilities.

Illinois Rep. Mike Quigley, a member of the panel, said Atkinson said that the complaint was "based on a series of events."

In calling the inspector general to testify, Schiff said Atkinson determined the whistleblower complaint was "credible and urgent" and should be "transmitted to Congress."

The inspector general's testimony was described by three people with knowledge of the proceedings. They were not authorized to discuss the meeting by name and were granted anonymity.

Several lawmakers suggested the failure to disclose the complaint's contents amounted to a failure to protect the whistleblower, another violation. However, the general counsel for the Office of the Director of National Intelligence, Jason Klitenic, wrote in a letter Tuesday to the committee that the agency was indeed protecting the whistleblower.

Andrew Bakaj, a former intelligence officer and an attorney specializing in whistleblower reprisal investigations, confirmed that he was representing the whistleblower but declined further comment.

Rep. Jim Himes, D-Conn., said 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, "We don't know exactly what is in the substance of this complaint. It could be nothing. It could be something very, very serious."