WASHINGTON • The effort to pressure Ukraine for political help provoked a heated confrontation inside the White House last summer that so alarmed Mr John Bolton, then the national security adviser, that he told an aide to alert White House lawyers, House investigators were told on Monday.

Mr Bolton got into a sharp exchange on July 10 with Mr Gordon Sondland, the Trump donor turned ambassador to the European Union, who was working with Mr Rudy Giuliani, US President Donald Trump's personal lawyer, to press Ukraine to investigate Democrats, according to three people in the room who heard the testimony.

Mr Bolton instructed Ms Fiona Hill, then the senior director for Russian and Eurasian affairs, to notify the chief lawyer for the National Security Council of a rogue effort by Mr Sondland, Mr Giuliani and Mr Mick Mulvaney, the acting White House Chief of Staff, with legal implications, Ms Hill told the investigators, according to the people familiar the testimony.

"I am not part of whatever drug deal Rudy and Mulvaney are cooking up," Mr Bolton, a Yale-trained lawyer, told Ms Hill to tell White House lawyers, according to two people familiar with the testimony.

It was not the first time Mr Bolton expressed grave concerns to Ms Hill about Mr Giuliani. "Giuliani's a hand grenade who's going to blow everybody up," she quoted Mr Bolton as saying during an earlier conversation.

The testimony revealed in a powerful way just how divisive Mr Giuliani's efforts to extract damaging information about Democrats from Ukraine on Mr Trump's behalf were within the White House.

Ms Hill testified that Mr Giuliani and his allies circumvented the usual national security process to run their own rump foreign policy, leaving the President's official advisers aware of the rogue operation yet powerless to stop it. Ms Hill was the first former White House official to testify in the impeachment inquiry, and her account provided a gripping in-the-room view of the shadow manoeuvres that have jeopardised Mr Trump's presidency.

While she left her post shortly before the now-famous July 25 phone call in which Mr Trump pressed Ukraine's President Volodymyr Zelensky to investigate Democrats, she helped House investigators understand the early months of the pressure campaign.

The day-long interview with Ms Hill came as House Democrats widened their net in the fast-paced inquiry by summoning Mr Michael McKinley, a senior adviser to Secretary of State Mike Pompeo who abruptly resigned last week, to testify today. Three other administration officials are scheduled to talk to investigators this week despite the White House's declaration last week that it would refuse to cooperate with the impeachment effort.

Ms Hill's testimony, which unfolded behind closed doors over nine hours in the secure rooms of the House Intelligence Committee on Capitol Hill, had been highly anticipated because of her position in a key job coordinating policy towards Russia, Ukraine and the rest of Europe. The House Intelligence Committee issued a last-minute subpoena on Monday morning to compel Ms Hill to speak to the investigators, according to an official involved in the investigation, to make it easier for her to justify ignoring the White House's clear opposition to cooperation with the House inquiry. Ms Hill testified that she opposed the idea of the July 25 phone call between Mr Trump and Mr Zelensky because she did not understand its purpose.

While it was described as a congratulatory call following parliamentary elections in Ukraine, Mr Trump had already made a congratulatory call to Mr Zelensky in April following his own election.

She was not told that Mr Trump would use the call to press for an investigation into his political rival, former US vice-president Joe Biden, nor did she know about the President's decision to withhold US$391 million (S$536 million) in US assistance to Ukraine until shortly before her departure, according to the person informed about her account.

Her testimony does not establish a quid pro quo between the suspended aid and Mr Trump's pressure for investigations, the person said. But she would confirm that the administration leveraged a coveted White House invitation for Mr Zelensky to a commitment to probe corruption, which was seen as code for investigating Democrats.