Photo: The Canadian Press

Former White House advisor on Russia, Fiona Hill, leaves Capitol Hill in Washington, Monday.

The impeachment inquiry is revealing vivid new details about the high-level unease over President Donald Trump's actions toward Ukraine, and those of his personal lawyer, Rudy Giuliani, as the swift-moving probe by House Democrats showed no signs Tuesday of easing.

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.

One witness, former White House aide Fiona Hill, testified that national security adviser John Bolton 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."

Another, 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.

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.

With Ukraine situated between the United States' Western allies and Russia, Pelosi noted the inquiry raises fresh questions about Trump's relationship with Russian President Vladimir Putin.

"All roads seem to lead to Putin with the president," she said.

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 appear 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.