Parnas, who helped Giuliani look for dirt on Trump's political rival, former Vice President Joe Biden, is a key figure in the impeachment inquiry that is examining whether Trump abused his office for personal political gain.

His apparent decision to now work with the congressional committees represents a change of heart. Parnas rebuffed a request from three House of Representatives committees last month to provide documents and testimony.

"We will honour and not avoid the committee's requests to the extent they are legally proper, while scrupulously protecting Mr Parnas' privileges including that of the Fifth Amendment," said the lawyer, Joseph Bondy, referring to his client's constitutional right to avoid self-incrimination.

His previous lawyer, John Dowd, wrote to the committees in early October complaining that their requests for documents were "overly broad and unduly burdensome".

Parnas pleaded not guilty in Manhattan federal court last month to being part of a scheme that used a shell company to donate money to a pro-Trump election committee and illegally raise money for a former congressman as part of an effort to have the president remove the US ambassador to Ukraine.

The indictment does not address the issues involved in the impeachment inquiry.

Parnas would be a crucial witness if he were to cooperate. He has said he played a key role in connecting Giuliani to Ukrainian officials during Giuliani's investigation into Biden and his son Hunter.

Trump's request to Ukraine's president, Volodymyr Zelenskiy, in a July 25 phone call to investigate the Bidens was at the heart of a whistleblower complaint by an intelligence officer that sparked the Democratic-led impeachment inquiry on September 24.

Two US diplomats told lawmakers the State Department was being used for domestic political purposes under Trump and warned that would hurt American interests, according to transcripts released on Monday from Congress' impeachment investigation.

Michael McKinley, a former top adviser to Secretary of State Mike Pompeo, told US House of Representatives committees that injecting domestic politics weakened diplomacy.

"I feel that this is not the way we maintain the integrity of the work we do beyond our borders. We're meant to project nonpartisanship overseas," he said.

The transcripts were the first released by the closed-door inquiry by the House Foreign Affairs, Intelligence and Oversight panels. They painted a detailed picture of the distress felt by top US diplomats as Trump allies tried to pressure Ukraine to launch investigations into the Republican president's political rivals.

McKinley said he quit his post after the State Department opted not to defend then-US Ambassador to Ukraine Marie Yovanovitch from attacks by Trump and his political allies.

He also cited his concern over "what appears to be the utilization of our ambassadors overseas to advance domestic political objectives," McKinley told lawmakers.

Yovanovitch was abruptly recalled as US ambassador to Ukraine after a smear campaign by Trump allies. In her testimony, also released on Monday, she said the State Department was being "attacked and hollowed out from within" and that not rebuilding it would harm the United States.

The committees began releasing interview transcripts as they prepare for public hearings this month.

Republicans have complained that the inquiry is insufficiently transparent, one of the main arguments of Trump's backers. Democrats say they are following House rules on investigations.

Trump has denied wrongdoing and accused Democrats of unfairly targeting him to reverse his surprise election victory in 2016.

Australian Associated Press