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.
What do we know about the complaint?
Only a few details about the complaint, which was filed on 12 August, 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.
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 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.
What sort of allegations have there been involving Ukraine?
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”.
They allege that Mr Trump and Mr Giuliani attempted to pressure the Ukrainian government into investigating former Vice-President Joe Biden and his son Hunter, who previously served on the board of a Ukrainian gas company.
Joe Biden is currently seeking the Democratic presidential nomination.
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Mr Trump and Ukrainian President Volodymyr Zelenskyy had a phone call on 25 July, two and a half weeks before the complaint was filed.