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.
However, the Washington Post and the New York Times both said at least part of the complaint related to Ukraine, citing officials with knowledge of the matter.
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.”