Rex Tillerson seen in Nairobi on Saturday.

Donald Trump has fired his secretary of state, Rex Tillerson, and announced his intention to replace him with the CIA director, Mike Pompeo.

Trump announced the shake-up in a tweet, adding that Gina Haspel, Pompeo’s deputy, would become the CIA’s first female director.

Tillerson’s departure had long been predicted after a series of clashes over policy. But the announcement, made just four hours after the secretary of state landed in Washington after a tour of Africa, took Tillerson unawares.

Under-secretary of state Steve Goldstein said: “The secretary did not speak to the president and is unaware of the reason, but he is grateful for the opportunity to serve, and still believes strongly that public service is a noble calling.”

Goldstein was later fired, the state department said.

Tillerson’s dismissal was immediate. State officials said they did not know if he would clear his office or send someone to do it for him. Officials said Tillerson had shown every intention of staying and had a full programme of commitments. Tillerson had recently predicted he would stay in office for all of 2018 at least.

At 9.15am, Trump left the White House for California, to see prototypes for his border wall. Pausing by the Marine One helicopter, he told reporters he had been “talking about this for a long time”.

“I’ll be speaking to Rex over a long period of time,” Trump said. “I actually got on well with Rex but it was a different mindset.”

He added: “When you look at the Iran deal: I think it’s terrible, I guess he thought it was OK. I wanted to either break it or do something and he felt a little bit differently.”

Tillerson has argued that the US should abide by the agreement about Tehran’s nuclear ambitions that was reached under Barack Obama in 2015. Pompeo is a longstanding opponent of the deal.

“With Mike,” Trump said, “we have a very similar thought process. I think it’s going to go very well.”

A graduate of West Point and Harvard and a former Republican congressman, Pompeo is widely seen as more of a loyalist than Tillerson, a former oil executive who had not met Trump before the election and grew increasingly at odds with his style and policies.

The president said: “I’ve worked with Mike Pompeo now for quite some time. Tremendous energy, tremendous intellect, we’re always on the same wavelength. The relationship has been very good and that’s what I need as secretary of state. I wish Rex Tillerson well.”

Last summer, Tillerson was reported to have called Trump a “fucking moron”, a report he did not deny.

Trump was asked twice if he had fired Tillerson “because he called you a moron?” The president twice said he could not hear the question, then said: “I respect his intellect. I respect the process that we’ve all gone through together. We have a very good relationship for whatever reason, chemistry, whatever it is – why do people get along?

“I’ve always, right from the beginning, from day one, I’ve gotten along well with Mike Pompeo, and frankly I get along well with Rex too. I wish Rex a lot of good things. I think he’s going to be very happy. I think Rex will be much happier now.”

On Monday, Tillerson issued a much stronger response to the attempted nerve agent assassination of a former Russian spy in the UK than the White House, naming Russia as a suspect, a step Trump’s spokeswoman, Sarah Sanders, had avoided.

Trump said he would speak to the British prime minister, Theresa May, on Tuesday. “It sounds to me like they believe it was Russia,” he said, adding that he would “take that finding as fact”.

Nancy Pelosi, the Democratic leader in the House, said: “President Trump’s actions show that every official in his administration is at the mercy of his personal whims and his worship of Putin.”

A senior White House official told reporters the firing was related to “upcoming talks with North Korea and various ongoing trade negotiations”. Responding to a question about his announcement of a meeting with Kim Jong-un shortly after Tillerson said talks were “a long way” off, Trump said: “No, I really didn’t discuss it very much with him, honestly.

“I made that decision by myself. Rex wasn’t, as you know, in this country. I made the North Korea decision with consultation from many people but I made that decision by myself.”

Chris Murphy, a Democrat on the Senate foreign relations committee, said Tillerson had “systematically and intentionally weakened American diplomacy” and added: “President Trump seems to want someone who does the same thing, only faster and while fawning over the president.”

In a statement, Pompeo said he was “deeply grateful to President Trump” and added: “His leadership has made America safer and I look forward to representing him and the American people to the rest of the world to further America’s prosperity.”