The Trump administration is blocking Congress from access to a whistleblower's reported complaint about a private conversation between President Donald Trump and a foreign leader.
The blocked complaint is both "serious" and "urgent," the government's intelligence watchdog said.
The administration is keeping Congress from learning what exactly the whistleblower is alleging.
One report said it involved a promise Mr Trump made in a phone call to a foreign leader.
A lawmaker said the complaint was "based on a series of events."
The intelligence community's inspector general appeared before the House intelligence committee behind closed doors on Thursday but declined, under administration orders, to reveal the substance of the complaint.
Mr Trump, though giving no details about any incident, denied on Thursday that he would ever "say something inappropriate" on such a call.
"Another Fake News story out there - It never ends!" Mr Trump tweeted.
"Virtually anytime I speak on the phone to a foreign leader, I understand that there may be many people listening from various US agencies, not to mention those from the other country itself. No problem!"
"Is anybody dumb enough to believe that I would say something inappropriate with a foreign leader while on such a potentially 'heavily populated' call."
Chairman of the Intelligence Committee, Adam Schiff Source: AAP
Democrat Adam Schiff, 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 Mr Schiff, describing the administration's blockade as an unprecedented departure from the law.
The standoff raised questions about the new acting director of national intelligence, Joseph Maguire, and whether he was working to shield the president from the reach of Congress.
Mr Maguire has been subpoenaed by the House panel and is expected to testify publicly next Thursday.
