U.S. President Donald Trump on Tuesday said he had replaced U.S. Secretary of State Rex Tillerson with Central Intelligence Agency (CIA) Director Mike Pompeo and had tapped Gina Haspel to lead the CIA.

"Mike Pompeo, Director of the CIA, will become our new Secretary of State. He will do a fantastic job! Thank you to Rex Tillerson for his service! Gina Haspel will become the new Director of the CIA, and the first woman so chosen. Congratulations to all!" Trump said on Twitter.

Speaking to reporters at the White House shortly before departing for Joint Base Andrews for his trip to California, Trump said that he made the decision to oust Secretary of State Rex Tillerson "by myself."

The president added that Tillerson will be "much happier now," and he appreciates his service. Trump said he and Tillerson had been "talking about this for a long time," and they had disagreed on issues like the Iran deal.

Trump also praised the energy and intellect of his incoming Secretary of State, Mike Pompeo, who has led the CIA.

In his first speech after he was fired by Trump, Tillerson said he was delegating all of his responsibilities to the State Department's No. 2 official and that he would leave office on March 31.

"What is most important is to ensure an orderly and smooth transition during a time that the country continues to face significant policy and national security challenges," Tillerson told reporters in a packed briefing room at the State Department.

Tillerson has long been dogged by reports that his time in office will be cut short. As recently as December, Trump had rejected speculation that Tillerson's days were numbered, noting that the two "work well together."

A White House official said Trump wanted to have a new team in place ahead of upcoming talks with North Korea and various trade talks. The official spoke on condition of anonymity because the official was not authorized to speak publicly.

"We got along actually quite well but we disagreed on things," Trump told reporters. "When you look at the Iran deal, I thought it was terrible, he thought it was okay. I wanted to either break it or do something, he felt a little differently. So we were not really thinking the same."

Friction between the president and the country's top diplomat has grown increasingly public through the year. For Tillerson, who left his job as Exxon Mobil's CEO, a premature departure from the cabinet had been described by media outlets as inevitable. At the White House, frustration with Tillerson has mounted over what officials have described as his aloofness and his slowness in filling key roles to carry out the president's agenda.

"There's been a Tillerson death watch since the spring," said Derek Chollet, a former State Department, Pentagon and National Security Council official in President Barack Obama's administration.

Trump and Tillerson have not always seen eye to eye on matters of foreign policy. On Monday Tillerson blamed Russia for the poisonings in England of a former Russian double agent and his daughter. Earlier, White House press secretary Sarah Sanders refrained from saying Moscow was responsible. When Tillerson in June called on Arab countries to ease their blockade on Qatar, Trump emerged in the Rose Garden hours later to lambaste Qatar for funding terrorism. Trump also deemed diplomacy with North Korea a waste of time when Tillerson was pursuing just that. Tillerson's advice to Trump to remain in the Paris climate deal and certify Iran's compliance with the nuclear deal was similarly overruled.

Pompeo, in contrast, has formed a tight relationship with Trump that has led to a role much broader than many past CIA directors. A former businessman and conservative Republican congressman from Kansas, Pompeo is at the White House nearly every day to deliver the daily intelligence briefing, a task often delegated to less senior officials. He sometimes stays longer to accompany Trump to other meetings. He shares the president's hardline stance against Iran.

Tillerson joined a long list of senior officials who have either resigned or been fired since Trump took office in January 2017. Others include strategist Steve Bannon, national security adviser Michael Flynn, FBI Director James Comey, White House chief of staff Reince Priebus, health secretary Tom Price, communications directors Hope Hicks and Anthony Scaramucci, and press secretary Sean Spicer. The latest to announce his exit is Gary Cohn, Trump's chief economic adviser, who had clashed with the boss over trade policy.

Cohn's departure has sparked internal fears of an even larger exodus, raising concerns in Washington of a coming "brain drain" around the president that will only make it more difficult for Trump to advance his already languishing policy agenda.

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