Donald Trump demanded on Monday that a whistleblower whose warning about the US president's call with Ukraine triggered the impeachment inquiry against him be identified and testify before Congress.

As the president menaced the person who exposed his potential wrongdoing, Trump faced a new setback with his former top Russia advisor, Fiona Hill, sitting for a closed-door deposition on Monday before Capitol Hill lawmakers.

Hill served in the National Security Council but left the administration shortly before Trump's July 25 call with his Ukrainian counterpart Volodymyr Zelensky.

Democrats expect her to share her concerns about Trump's involvement in the Ukraine scandal, including his ouster of the US ambassador to Kiev Marie Yovanovitch, who testified to Congress last week.

Hill, under a congressional subpoena according to her lawyer, made no remarks to reporters as she entered the secure meeting room in the US Capitol for a deposition expected to last several hours.

With the impeachment inquiry charging ahead, Trump lashed out at House Intelligence Committee chairman Adam Schiff after the Democratic lawmaker suggested the whistleblower might not testify out of concern for their safety.

Trump has repeatedly pushed for the unmasking of the unidentified author of a complaint that said Trump may have abused his power on the call by urging Ukraine to dig up dirt on his political rival Joe Biden.

Whistleblowers are protected by US law, and revealing their identity is a crime.

"Adam Schiff now doesn't seem to want the Whistleblower to testify. NO! Must testify to explain why he got my Ukraine conversation sooo wrong," Trump tweeted.

"Did Schiff tell him to do that? We must determine the Whistleblower's identity to determine WHY this was done to the USA."

Trump repeatedly characterises his Zelensky call as "perfect," but the whistleblower's complaint noted how some White House officials were so concerned about Trump's actions on the call that they sought to severely restrict access to its record.

The White House memo of the call shows Trump sought a "favor" from Zelensky. Democrats say it was a demand to investigate Joe Biden - the president's potential 2020 election rival - and a Ukrainian firm that hired Biden's son Hunter.

House Democrat Jamie Raskin said he believes Hill could shed light on what he described as "a very powerful shadow foreign policy being operated out of Ukraine by the president's lawyer Rudy Giuliani" that bypassed normal channels in order to apply coercive pressure on Kiev.

"I think that she would have a comprehensive overview of that whole situation," Raskin told MSNBC.