Donald Trump's personal lawyer Rudy Giuliani dove headfirst Tuesday into a war of words with John Bolton, caling the former national security adviser 'an atomic bomb' behind the scenes of the White House – dramatically raising the rhetorical stakes after Bolton called him 'a hand grenade.'

Efforts in the administration to pressure Ukraine into investigating Trump's Democratic rival Joe Biden so alarmed Bolton that he alerted a lawyer, his former aide Fiona Hill testified Monday in a congressional deposition behind closed doors.

Hill told lawmakers that Bolton had said Giuliani, the president's point-man on Ukraine, was 'a hand grenade who's going to blow everyone up,' according to an account in The New York Times.

Giuliani fired back in a statement Tuesday that dripped with sarcasm: 'I always liked and respected John. I'm very disappointed that his bitterness drives him to attack a friend falsely and in a very personal way. It's really ironic that John Bolton is calling anyone else a hand grenade. When John is described by many as an atomic bomb.'

John Bolton (right), the former national security adviser, was said to be so alarmed at White House efforts to pressure Ukraine for dirt on Democrats that he told a White House aide to alert lawyers, it is being reported

Following a 'sharp exchange' with U.S. ambassador to the European Union Gordon Sondland, who was working with Giuliani on the pressure campaign, Bolton instructed Hill to notify a National Security Council lawyer, according to the Times.

Referring to acting White House chief of staff Mick Mulvaney, Bolton told Hill to relay a message to the chief lawyer for the National Security Council: 'I am not part of whatever drug deal Sondland and Mulvaney are cooking up.'

Sondland, a businessman who donated $1 million to Trump's inaugural committee, is the U.S. ambassador to the European Union. He worked in tandem with Giuliani on the Ukraine portfolio.

The Times reported that Bolton had a sharp exchange with Sondland on July 10 about White House efforts to pressure Ukraine to investigate Democrats.

Hill's testimony followed allegations that Trump pushed Ukrainian President Volodymyr Zelensky during a July 25 phone call to investigate the Bidens, sparking a scandal that has led to an impeachment inquiry by Democrats in the House of Representatives.

The White House's transcript of the call shows Trump sought a 'favor' from Zelensky.

Bolton was said to be worried about Rudy Giuliani (seen right with associate Lev Parnas), who he called a 'hand grenade.' Parnas and another associate, Igor Fruman, have been arrested on campaign finance charges

Democrats who control the House say Trump and others were demanding Ukraine investigate Biden, the president's potential 2020 election rival, and a Ukrainian firm that hired Biden's son Hunter.

On Monday evening, Giuliani said, 'I don't know Fiona and can't figure out what she is talking about,' and that the State Department arranged all his contacts with Ukraine.'

The Times reported last week that Giuliani was himself under federal investigation for his dealings with Kiev on Trump's behalf.

Trump in September fired Bolton, who is known for his hardline stances on Washington's archenemies, particularly Iran, North Korea and Venezuela.

Hill left the Trump administration shortly before the July call with Zelensky.

Giuliani, acting White House chief of staff Mick Mulvaney (left), and U.S. Ambassador to the EU Gordon Sondland (right) were said to be involved in efforts to pressure Ukraine

She told House impeachment investigators behind closed doors Monday that she had strongly and repeatedly objected to the ouster earlier this year of former ambassador to Ukraine Marie Yovanovitch, according to a person familiar with the testimony.

Yovanovitch testified Friday that President Trump pressured the State Department to fire her.

The interview is one of what could eventually become dozens of closed-door depositions in the impeachment probe.

There are five more scheduled this week, mostly with State Department officials, though it is unclear if they will all appear after Trump declared he wouldn't cooperate with the probe.

While interviews have focused on the interactions with Ukraine, the probe could broaden as soon as next week to include interviews with White House budget officials who may be able to shed light on whether military aid was withheld from Ukraine as Trump and his lawyer, Giuliani, pushed for the investigations.

The three committees leading the probe are seeking interviews next week with Russell Vought, acting director of the Office of Management and Budget, and Michael Duffey, another OMB official who leads national security programs, according to a person familiar with those requests.

That person wasn't authorized to discuss the invitations and requested anonymity.

The packed schedule of interviews comes as Democrats are methodically working to pin down the details of Trump's pressure on Zelensky.

Once Democrats have completed the probe, and followed any other threads it produces, they will use their findings to help determine whether to vote on articles of impeachment.

House Speaker Nancy Pelosi said she wants the committees to move 'expeditiously.'

Fiona Hill (center), a former White House expert on Russia, testified before Congress for more than 10 hours on Monday. She said she was asked by Bolton to alert White House lawyers about Ukraine

Democrats have already obtained documents and testimony that verify parts of an original whistleblower's complaint that launched the probe.

A cache of text messages between three diplomats provided by one of the inquiry's first witnesses, former Ukrainian envoy Kurt Volker, detailed attempts by the diplomats to serve as intermediaries around the time Trump urged Zelensky to start the investigations into a company linked to Biden's son.

Yovanovitch told lawmakers there was a 'concerted campaign' against her based on 'unfounded and false claims by people with clearly questionable motives.'

One of the diplomats in the text exchanges, Sondland, is expected to appear for a deposition under subpoena Thursday.

He's expected to tell Congress that his text message reassuring another envoy that there was no quid pro quo in their interactions with Ukraine was based solely on what Trump told him, according to a person familiar with his coming testimony.

Also up this week: Michael McKinley, a former top aide to Secretary of State Mike Pompeo who resigned last week.

McKinley, a career foreign service officer and Pompeo's de facto chief of staff, resigned Friday, ending a 37-year career.

He is scheduled to testify behind closed doors Wednesday.

The committees are also scheduled to talk to Deputy Assistant Secretary of State George Kent on Tuesday and Ulrich Brechbuhl, a State Department counselor, on Thursday.

On Friday, the lawmakers have scheduled an interview with Laura Cooper, who is the Deputy Assistant Secretary of Defense for Russia, Ukraine and Eurasia.

It is unclear if any of those officials will show up after Trump's vow of non-cooperation.

President Trump is said to have wanted Ukraine's newly elected leader, Volodymyr Zelensky (left), to offer up dirt on Joe Biden

Because of the Trump administration's edict, the Democrats have been subpoenaing witnesses as they arrived for their interviews - a move sometimes known as a 'friendly' subpoena that could give the witnesses additional legal protection as they testify.

Both Yovanovitch and Hill received subpoenas the mornings of their testimony.

One witness who may not be called before Congress is the still-anonymous government whistleblower who touched off the impeachment inquiry.

Top Democrats say testimony and evidence coming in from other witnesses, and even the Republican president himself, are backing up the whistleblower's account of what transpired during Trump's July 25 phone call with Zelensky.

Lawmakers have grown deeply concerned about protecting the person from Trump's threats and may not wish to risk exposing the whistleblower's identity.

House Intelligence Committee Chairman Adam Schiff, D-Calif., said Sunday it 'may not be necessary' to reveal the whistleblower's identity as the House gathers evidence.

He said Democrats 'don't need the whistleblower, who wasn't on the call, to tell us what took place on the call.'

When Joe Biden was vice president, his son, Hunter (right), served on the board of a Ukrainian gas company that was being investigated for corruption

Schiff said the 'primary interest right now is making sure that that person is protected.'

Trump showed no signs of backing down.

'Adam Schiff now doesn't seem to want the Whistleblower to testify. NO!' the Republican president tweeted Monday.

'We must determine the Whistleblower's identity to determine WHY this was done to the USA.'

Republican lawmakers have aimed their ire at Democrats and the process, saying Pelosi should hold a vote to begin the inquiry and hold the meetings out in the open, not behind closed doors.

'The tragedy here and the crime here is that the American people don't get to see what's going on in these sessions,' said Ohio Rep. Jim Jordan, the top Republican on the House Oversight and Reform panel.