Former US national security adviser John Bolton was so alarmed about White House efforts to pressure Ukraine, he told an aide to alert a lawyer, US media report.
Mr Bolton also reportedly warned that Mr Trump's personal lawyer, Rudy Giuliani, was "a hand grenade who's going to blow everyone up".
Fiona Hill, a former official, told US lawmakers about the remarks on Monday.
She testified behind closed doors as part of the impeachment inquiry.
The probe, led by US Democrats, is looking into whether Mr Trump improperly pressured Ukraine to investigate Joe Biden, his political rival.
The inquiry stems from a whistleblower complaint about a July phone call between the Republican president and his Ukrainian counterpart Volodymyr Zelensky.
Ms Hill, who served as the senior director for European and Russian affairs on the National Security Council, is the first former White House official to testify in the impeachment inquiry. She left the Trump administration shortly before the 25 July call with Mr Zelensky.
A rough transcript of the call released by the White House shows Mr Trump asked Mr Zelensky to investigate Mr Biden, now a Democratic frontrunner for next year's White House election.
But Mr Trump denies any wrongdoing, or that he withheld military aid in order to pressure Ukraine.
What did Hill testify?
Fiona Hill's testimony, as reported by the New York Times, Politico and NBC News, revealed what Mr Bolton thought of an effort by some White House officials to pressure Ukraine.
Ms Hill said Mr Bolton told her to discuss the matter with the top lawyer at the National Security Council after he had a testy exchange on 10 July with Gordon Sondland, the US ambassador to the EU.
Mr Sondland was involved in the Ukraine pressure campaign, along with Mr Giuliani. Mick Mulvaney, the acting White House chief of staff, was also seen as a key player, according to Ms Hill's testimony.
Mr Bolton, who left the White House in September, said he wanted no part in their scheme, Ms Hill told lawmakers.
"I am not part of whatever drug deal Rudy and Mulvaney are cooking up," Ms Hill said Mr Bolton told her, according to Politico. The New York Times reports the same line but says Ms Hill said Mr Bolton referred to Mr Sondland and Mr Mulvaney, not Mr Giuliani and Mr Mulvaney.
Responding to the testimony on Monday, Mr Giuliani said, "I don't know Fiona and can't figure out what she is talking about", according to the Washington Post.
Who else has been called to testify?
Last week Marie Yovanovitch, the former US ambassador to Ukraine, told lawmakers she was sacked over "unfounded and false claims by people with clearly questionable motives".
Ms Yovanovitch said she was "incredulous" at being dismissed by President Trump in May. The decision to dismiss her reportedly followed Mr Giuliani and other conservatives arguing she was biased against the president.
Earlier this month, House Democrats also heard from former US special envoy to Ukraine, Kurt Volker.
Before he testified, Mr Volker handed over text messages between US diplomats documenting the Trump administration's efforts to have Ukraine investigate the energy company that employed Mr Biden's son when his father was Barack Obama's vice-president.
Last week, Mr Sondland confirmed he would comply with a House subpoena to face the Democratic-led committees on 17 October.