Former national security adviser John Bolton was so alarmed by Rudy Giuliani's pressure campaign on Ukraine for dirt on former Vice President Joe Biden that he directed a top aide to tell White House lawyers he wasn't involved, The New York Times reported.

Bolton's directive came after he got into a heated exchange with Gordon Sondland, the US ambassador to the European Union who was part of Giuliani's shadow foreign policy with respect to Ukraine.

"I am not part of whatever drug deal Rudy and Mulvaney are cooking up," Bolton instructed his aide, Fiona Hill, to tell White House lawyers, according to Hill's testimony cited by The Times.

The Times also reported that Bolton described Giuliani as a "hand grenade" that would "blow everybody up."

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John Bolton, the former national security adviser, was so alarmed by Rudy Giuliani's efforts to pressure Ukraine for dirt on former Vice President Joe Biden that he told his top aide to inform White House lawyers he wasn't part of it, The New York Times reported.

The former aide who previously served as a senior director for Russian and Eurasian affairs, Fiona Hill, testified to House Democrats on Monday that Bolton instructed her to tell the National Security Council's chief lawyer that Giuliani and acting White House chief of staff Mick Mulvaney were carrying out an operation with serious legal implications, The Times reported.

Bolton's directive came after he got into a heated July 10 exchange with Gordon Sondland, the US ambassador to the European Union who was part of Giuliani's shadow foreign policy with respect to Ukraine.

"I am not part of whatever drug deal Rudy and Mulvaney are cooking up," Bolton told Hill to tell White House lawyers, according to Hill's testimony cited by The Times.

The Times also reported that Bolton described Giuliani as a "hand grenade" that would "blow everybody up."

CNN's Jim Acosta also reported, citing a source familiar with Hill's testimony, that she witnessed "wrongdoing" and tried to bring it to the attention of White House lawyers.

Bolton was ousted as national security adviser last month after a tumultuous tenure at the White House. Hill, meanwhile, joined the White House as its top Russia analyst in April 2017 and stepped down in August.

Hill's testimony Monday was part of Congress' brewing impeachment inquiry examining whether President Donald Trump used his public office for private gain. At the center of the investigation is a July 25 phone call Trump had with Ukrainian President Volodymyr Zelensky, during which Trump repeatedly pressed Zelensky to investigate the Bidens and help discredit the FBI's Russia investigation.

Giuliani's involvement in pressuring Ukraine goes back much further; Giuliani, in particular, has worked gather dirt on the Bidens since at least spring of last year, according to reports.

A whistleblower complaint filed in August by an intelligence official has become the center of the impeachment inquiry, which is investigating Giuliani and the Trump administration's efforts in Ukraine and whether Trump tried to leverage US foreign policy in exchange for information that would personally benefit his reelection campaign.