Donald Trump has denied a report alleging he made a promise to a foreign leader, which sparked a whistleblower’s formal complaint.
The complaint is reported to relate to a July phone call with Ukraine’s new president, Volodymyr Zelensky.
The Washington Post said an 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.
However, some reports allege that Mr Trump asked Mr Zelensky to investigate Joe Biden and his son Hunter – who previously served on the board of a Ukrainian gas company – in return for continued US military support.
Joe Biden is currently the frontrunner to be the Democratic candidate competing against Mr Trump in the 2020 presidential election.
It was filed on August 12 and only a few details have been made public.
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 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.
Under US law, if the complaint is considered to be of “urgent concern”, and if the inspector general considers the complaint to be “credible”, then the department head is expected to share the information with Congress within seven days.
Earlier this month, before the whistleblower’s complaint came to light, House Democrats launched an investigation into Mr Trump and his lawyer Rudy Giuliani’s interactions with Ukraine.
Three Democratic panel heads – Eliot Engel (foreign affairs), Adam Schiff (intelligence) and Elijah Cummings (oversight) – said Mr Trump and Mr Giuliani had attempted “to manipulate the Ukrainian justice system to benefit the president’s re-election campaign and target a possible political opponent”. -BBC