US    -    Washington (CNN) President Donald Trump abruptly announced in a tweet Tuesday that he has asked national security adviser John Bolton to resign, noting that he “strongly disagreed with many” of Bolton’s suggestions “as did others in the administration.”

“I thank John very much for his service. I will be naming a new National Security Advisor next week,” Trump wrote.

The tweet came just one hour after the White House press office said Bolton was scheduled to appear at a Tuesday press briefing alongside Secretary of State Mike Pompeo and Treasury Secretary Steve Mnuchin.

Asked during the briefing whether he and Mnuchin were surprised that Bolton was fired, given that he was supposed to appear alongside them, Pompeo said, “I’m never surprised.”

Bolton, however, maintains he offered to leave his post voluntarily, tweeting minutes after Trump’s announcement, “I offered to resign last night and President Trump said, ‘Let’s talk about it tomorrow.’” Bolton reiterated the point that he offered to resign to Fox News Tuesday.

His resignation letter to Trump, dated Tuesday, was short and to the point with Bolton writing: ‘I hereby resign, effective immediately, as Assistant to the President for National Security Affairs. Thank you for having afforded me this opportunity to serve our country.”

The White House is already discussing potential candidates to replace Bolton, several sources told CNN. There are at least 10 names being circulated and there does not appear to be a shortlist at this time. Trump said he will make a decision next week but given the President’s unpredictable nature, sources warn that he could choose someone who is not under discussion right now.

Trump has plowed through an unprecedented number of national security professionals while multiple geopolitical crises have played out.

The President has had three national security advisers -- Bolton, Michael Flynn and H.R. McMaster. He has summarily fired a secretary of state, Rex Tillerson, by tweet after undercutting the former ExxonMobil CEO for months.

Secretary of Defense James Mattis resigned, reportedly in frustration over Trump’s decision to pull out of Syria.

The President has also churned through two Homeland Security secretaries, John Kelly and Kirstjen Nielsen, and a National Security Agency director, Mike Rogers. He’s lost a deputy national security adviser, K.T. McFarland and an ambassador to the UN, Nikki Haley. Director of National Intelligence Dan Coats and his deputy, Sue Gordon, left their posts last month.

Bolton’s departure comes as tensions with Iran are escalating in the Persian Gulf, North Korea continues to develop its weapons capabilities, arms control experts are warning of a potential nuclear arms race with Russia and trade tensions with China are intensifying, while Trump is discussing a drawdown of forces in Afghanistan.

“John Bolton’s priorities and policies just don’t line up with the President’s and any sitting president has the right to put someone in that position that can carry out his agenda. That became no longer tenable so the President made a change,” Gidley told reporters.

He claimed there was “no one issue” that led to Bolton’s firing, and referred reporters to the forthcoming briefing for more information. Yet, Bolton’s ouster was so sudden that the now-former National Security Adviser even led a meeting of top administration officials, known as a principals committee meeting, Tuesday morning prior to Trump’s tweet, a source familiar told CNN.

The source said the meeting went on as planned and there was no indication that Bolton’s firing was imminent.

The campaign by Trump allies to push Bolton out of the administration had ramped up in recent weeks, multiple sources told CNN.

The President had received multiple phone calls and appeals to replace Bolton with someone who agreed with him more and was willing to follow through with his decisions when he didn’t. This was something Trump heard since he hired Bolton — but people noticed the President’s frustration with him was growing in recent weeks so they upped the pressure.

A senior administration official echoed that sentiment, telling CNN that Trump has been getting more and more irritated with Bolton over the past several months for his statements on Iran, Venezuela and now Afghanistan.

Trump no longer believed Bolton could advocate for the President’s agenda, and instead felt he was harming his credibility, the official said.

Bolton had recently expressed reluctance to defend the Trump administration on television. He was soft-booked to do “Meet the Press” and “This Week” on August 25 during the G7 summit, according to people familiar with the matter, but he later backed out.