WASHINGTON -- The House impeachment inquiry is exposing new details about unease in the State Department and White House about U.S. President Donald Trump's actions toward Ukraine and those of his personal lawyer Rudy Giuliani.

One witness said it appeared "three amigos" tied to the White House had taken over foreign policy. Another quoted national security adviser John Bolton as calling Giuliani a "hand grenade" for his back-channel efforts to get Ukraine to investigate Trump's Democratic rival Joe Biden and Biden's son Hunter.

On Wednesday, a former aide to Secretary of State Mike Pompeo is to speak to the House impeachment panels behind closed doors. Michael McKinley, who resigned last week, is a career foreign service officer and was Pompeo's de facto chief of staff.

He is expected to discuss concerns held by career State Department officials about the treatment of the U.S. ambassador to Ukraine, Marie Yovanovitch, and others who worked on the Ukraine portfolio, according to a person familiar with his testimony. A Latin America specialist, McKinley wasn't directly involved in Ukraine policy, but as a senior adviser to Pompeo was generally aware of the situation, the person said.

McKinley expects to talks about demoralization in the ranks of career foreign service officers and what many have lamented as the politicization of the once-apolitical bureaucracy, according to the person who spoke on condition of anonymity.

The 37-year veteran of the diplomatic corps was known to be unhappy with the state of affairs although his farewell note to colleagues mentioned nothing about the reason for his departure other than it was a "personal decision."

The testimony from the witnesses, mainly officials from the State Department and other foreign policy posts, is largely corroborating the account of the government whistleblower whose complaint first sparked the impeachment inquiry, according to lawmakers attending the closed-door interviews.

Trump's July 25 phone call in which he pressed Ukraine's president to investigate the Bidens is at the centre of the Democrats' inquiry.

Speaker Nancy Pelosi, despite intensifying calls from Trump and Republicans to hold a formal vote to authorize the impeachment inquiry, showed no indication she would do so. She said Congress will continue its investigation as part of the Constitution's system of checks and balances of the executive.

"This is not a game for us. This is deadly serious. We're on a path that is taking us, a path to the truth," Pelosi told reporters after a closed-door session with House Democrats.

Democratic leaders had been gauging support for a vote to authorize the impeachment inquiry after Trump and Republicans pushed them for a roll call. Holding a vote would test politically vulnerable Democrats in areas where the Republican president is popular.

Trump calls the impeachment inquiry an "illegitimate process" and is blocking officials from co-operating.

The inquiry is moving quickly as a steady stream of officials appears behind closed doors this week, some providing new revelations about the events surrounding the July 25 phone call between Trump and Ukrainian President Volodymyr Zelenskiy. It is on that call that Trump urged Zelenskiy to investigate a firm tied to political rival Joe Biden's family and Ukraine's own involvement in the 2016 presidential election.

Career State Department official George Kent testified Tuesday he was told by administration officials to "lay low" on Ukraine as "three amigos" tied to the White House took over U.S. foreign policy toward the Eastern European ally.

Kent was concerned about the "fake news smear" against Yovanovitch, whom Trump recalled in May, according to emails obtained by The Associated Press.

Kent told the lawmakers that he "found himself outside a parallel process" and had warned others about Giuliani as far back as March. He felt the shadow diplomacy was undermining decades of foreign policy and the rule of law in Ukraine and that was "wrong," said Rep. Gerry Connolly, D-Va.

Connolly said Kent described a May 23 meeting at the White House, organized by Trump's acting chief of staff, Mick Mulvaney, where three administration officials -- U.S. ambassador Gordon Sondland, special envoy Kurt Volker and Energy Secretary Rick Perry -- declared themselves the people now responsible for Ukraine policy.

"They called themselves the three amigos," Connolly said Kent testified.

Kent also told them that Trump, through the Office of Management and Budget, which Mulvaney previously led, was holding up military aid to Ukraine while pressing Zelenskiy to investigate a company linked to Biden's son.

"He was clearly bothered by the role Mr. Giuliani was playing," Connolly said.

Another witness, former White House aide Fiona Hill, testified Monday that Bolton, then national security adviser, was so alarmed by Giuliani's back-channel activities in Ukraine that he described him as a "hand grenade who is going to blow everybody up."

In 10 hours of testimony, Hill, the former White House aide who was a top adviser on Russia, recalled to investigators that Bolton had told her he was not part of "whatever drug deal Sondland and Mulvaney are cooking up," an apparent reference to talks over Ukraine.

She testified that Bolton asked her to take the concerns to National Security Council lawyer John Eisenberg.

As White House lawyers now try to learn more about the handling of the Ukraine call, Eisenberg is coming under particular scrutiny, said one White House official. He was both the official who ordered that the memorandum of the call be moved to a highly-classified system and the one who involved the Justice Department in a complaint from the CIA general counsel. The latter caught the attention of the president, according to the official.

Giuliani has acknowledged he had received payments totalling $500,000 related to the work for a company operated by Lev Parnas -- who, along with associate Igor Fruman, played a key role in Giuliani's efforts to launch a Ukrainian corruption investigation against Biden and his son Hunter. The two men were arrested last week on campaign finance charges as they tried to board an international flight.

Trump's team won't comply with the Democratic inquiry. Giuliani and Vice-President Mike Pence became the latest officials refusing to co-operate, saying through their lawyers they would not provide information requested by House Democrats as part of the impeachment inquiry.

The Defence Department said in a letter to the panel Tuesday it, too, could not comply with the request for documents. Among the grounds it cited was that the House hadn't taken a formal vote authorizing the impeachment inquiry.

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Associated Press writers Zeke Miller, Michael Balsamo, Eric Tucker, Matthew Lee, Padmananda Rama, Andrew Taylor and Alan Fram in Washington and Jonathan Lemire in Dallas contributed to this report.