Donald Trump has denied a report alleging he made a promise to a foreign leader, something that sparked a whistleblower’s formal complaint.
The Washington Post said the intelligence official found the comment “so troubling” they went to the department’s inspector general.
In a tweet, Mr Trump dismissed the claims as “fake news”.
Democrats are trying to get the complaint turned over to Congress, with the details still unknown.
Intelligence Community Inspector General Michael Atkinson said the complaint consists of a “serious or flagrant problem, abuse or violation of the law” that involves classified information, a letter to lawmakers revealed.
It is not yet known who made the complaint, which foreign leader Mr Trump was speaking to, what promise – if any – was made and whether Mr Trump took any action as a result.
The complaint was filed on 12 August, and Mr Trump had spoken by phone with Ukrainian President Volodymyr Zelenskyy two and a half weeks before that.
House Democrats have been investigating whether Mr Trump and his lawyer Rudy Giuliani tried to pressure the Ukrainian government to investigate former Vice-President Joe Biden, and his son Hunter Biden, who previously served on the board of a Ukrainian gas company.
Joe Biden is currently seeking the Democratic presidential nomination.
On Thursday afternoon, Mr Trump wrote on Twitter that he knew all his phone calls to foreign leaders were listened to by US agencies.
“Knowing all of this, is anybody dumb enough to believe that I would say something inappropriate with a foreign leader,” he asked.
Acting Director of National Intelligence Joseph Maguire has so far refused to share any details of the complaint with lawmakers, leading to an outcry among Democrats.
House Intelligence Chairman Adam Schiff, a Democratic California congressman, praised the whistleblower for coming forward after a closed hearing with Mr Atkinson on Thursday.
He also accused the White House and justice department of intervening to block the complaint from being shared with Congress.
“I believe that there is an effort to prevent this information getting to Congress,” said Mr Schiff.
“And if the assertion is accurate… then at one level or another, it likely involves either the president or people around him.”
Trump vs spies
Analysis by Tara McKelvey, White House reporter
Many of those who work in the US intelligence services don’t trust Trump. They don’t think that he can keep a secret (he apparently revealed classified information to Russians in the Oval Office), and they don’t like the way he talks about them (in a tweet about a news leak, he compared them to Nazis).
The controversy over the president’s “promise” to a foreign leader during a phone call – and a whistleblower complaint about the matter – is a reminder of the acrimonious relationship between the president and those who work in the intelligence services.
Yet like much of what happens in the world of intelligence, the matter is shrouded in mystery. It’s still unclear whom the president spoke to, for example, or what he said. In this case, though, some of these questions will be addressed during a House intelligence committee scheduled for next week, and more information may be available to the public.
As Trump himself would say: stay tuned.