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.
What do we know about the complaint?
It was filed on 12 August 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.
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 tried to pressure the Ukrainian government into investigating Joe and Hunter Biden.
Mr Trump and President Zelensky had a phone call on 25 July, two and a half weeks before the complaint was filed.
The two leaders will hold talks next week in New York, where Mr Zelensky will be attending the UN General Assembly, his office said.
Is Ukraine's leader being punished?
Analysis by Jonah Fisher, BBC Kiev correspondent
No-one in Ukraine's presidential administration wants to talk about the 25 July phone call between Mr Trump and Mr Zelensky, or respond to the claims that Mr Trump asked Ukraine to investigate his possible opponent in 2020 in return for continued US military support. There are a few circumstantial clues.
Firstly, the Ukrainian readout of the call refers to Mr Trump being "convinced" that the new government will "complete (the) investigation of corruption cases, which inhibited the interaction between Ukraine and the USA".
Is that a diplomatic reference to the US president asking for a probe into Mr Biden?
There's then the question as to why President Trump has kept Ukraine's new leader at arms' length since his election in April.
Some $250m in US military assistance for Ukraine was mysteriously held up by the White House this summer, and a long-mooted visit for President Zelensky to Washington has still not materialised.
Is Ukraine's leader being punished for refusing to play ball?
How have Trump and Giuliani responded?
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.
Another Fake News story out there - It never ends! Virtually anytime I speak on the phone to a foreign leader, I understand that there may be many people listening from various U.S. agencies, not to mention those from the other country itself. No problem!— Donald J. Trump (@realDonaldTrump) September 19, 2019
On Friday, Mr Trump tweeted that the conversation was "perfectly fine and respectful", without providing more details about who he spoke to. He also said the statement from the whistleblower was "highly partisan".
Meanwhile, Mr Giuliani gave an interview to CNN where he both confirmed, and denied, that he had asked Ukraine to investigate Joe Biden.
He explained the contradiction by saying that he had asked Ukraine "to look into the allegations that related to my client, which tangentially involved Joe Biden in a massive bribery scheme".
He also wrote on Twitter that "a President telling a Pres-elect of a well known corrupt country he better investigate corruption that affects US is doing his job".
Mr Trump and his fellow Republicans have questioned whether it represented a conflict of interest that Hunter Biden had served on the board of Burisma, a Ukrainian gas company.
In May, Ukraine's prosecutor general said there was no evidence of wrongdoing by Joe or Hunter Biden.
WATCH: Rudy Giuliani denies that he asked Ukraine to investigate Joe Biden, before admitting to Chris Cuomo seconds later: "Of course I did!" pic.twitter.com/flcierpKGu— Axios (@axios) September 20, 2019
Why hasn't the whistleblower's complaint been shared?
There is a disagreement between the acting director of national intelligence (DNI), Joseph Macguire, and Inspector General Michael Atkinson, who is responsible for conducting independent audits and investigations across the intelligence community.
A lawyer for Mr Macguire's office said in a letter to lawmakers that the complaint "does not meet the definition of urgent concern".
The complaint "concerned conduct by someone outside the intelligence community and did not relate to any 'intelligence activity' under the DNI's supervision", the letter said.
It also involved "potentially privileged matters", the letter added.
Mr Atkinson told lawmakers in a letter that he disagreed with the DNI's assessment, and believed the complaint "relates to one of the most significant and important of the DNI's responsibilities to the American people".
However, Mr Atkinson added that he was bound by the acting director of national intelligence's decision.