House investigators are summoning top Trump officials, including former national security adviser John Bolton, to testify next week as the impeachment probe pushes ever deeper into the White House.

Investigators want to hear from Mr Bolton, the hawkish former adviser who was openly troubled by President Donald Trump’s approach to Ukraine. He has since left the administration.

The Democrats are also calling John Eisenberg, the lawyer for the NSC who fielded an Army officer’s concerns over Mr Trump, and Michel Ellis, another security council official. Mr Eisenberg and Mr Ellis are being asked to appear on Monday with Mr Bolton on Thursday (local time), according to a person familiar with the planning and granted anonymity to discuss it.

The White House has urged officials not to testify in the impeachment proceedings and it’s unclear if these people will appear before the House panels.

The impeachment inquiry, now in its second month, is accelerating and deepening as the House prepares to take its first official vote Thursday (local time) on the process. Then come public hearings and the possible drafting of articles of impeachment against the president.

The investigation is focused on Trump’s July phone call with Ukraine when he asked President Volodymyr Zelenskiy to investigate Democrats and a potential 2020 political rival, Joe Biden, as the White House was withholding military aid Ukraine relies on for its defences.

On Wednesday, the Democratic and Republican House lawmakers heard fresh testimony about the Trump administration’s unusual back channels to Ukraine.

Two State Department Ukraine experts offered new accounts of Trump’s reliance on his personal attorney, Rudy Giuliani, rather than career diplomats, to engage with the East European ally, a struggling democracy facing aggression from Russia.

Foreign Service officer Christopher Anderson testified that Bolton cautioned him that Mr Giuliani “was a key voice with the president on Ukraine” and could complicate US goals for the country.

Another Foreign Service officer, Catherine Croft, said that during her time at the White House’s National Security Council, she received “multiple” phone calls from lobbyist Robert Livingston – a former top Republican lawmaker once in line to become House speaker – telling her the US Ambassador to Ukraine, Marie Yovanovitch, should be fired.

“It was not clear to me at the time – or now – at whose direction or at whose expense Mr. Livingston was seeking the removal of Ambassador Yovanovitch,” she said in prepared remarks.

Mr Livingston characterised Ms Yovanovitch as an “‘Obama holdover and associated with George Soros,” she said, referring to the American financier who is often the subject of conservative criticism in the US and Europe.

Most Democrats are expected to support the formal impeachment investigation resolution, even if they don’t back impeachment itself, saying they are in favour of opening the process with more formal procedures.

Public hearings are expected to begin in mid-November, a matter of weeks. Democrats are eager to hear from some top witnesses who have already provided compelling testimony behind closed doors, including diplomat William Taylor, a top ambassador in Ukraine, and Alexander Vindman, the Army officer who testified Tuesday (local time) that he twice reported to superiors his concerns about Trump’s actions toward Ukraine.

-AAP