Outgoing US Secretary of State Rex Tillerson has said he will be leaving office at the end of the month.

He was fired earlier today by US President Donald Trump after a series of public rifts over policy on North Korea, Russia and Iran.

Mr Trump announced that CIA Director Mike Pompeo will be appointed the new Secretary of State.

Speaking at a press conference, Mr Tillerson said that he was delegating all of his responsibilities to the State Department’s No. 2 official, Deputy Secretary John Sullivan .

"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," Mr Tillerson said.

Mr Tillerson warned that Washington must do more to respond to Russia's "troubling behavior and actions."

"Much work remains to respond to the troubling behavior and actions on the part of the Russian government," he said, before turning to warn Vladimir Putin's Kremlin not to overstep.

"Russia must assess carefully as to how its actions are in the best interests of the Russian people and of the world more broadly.

"Continuing on their current trajectory is likely to lead to greater isolation on their part, a situation which is not in anyone's interest."

The biggest shakeup of Mr Trump's cabinet since he took office in January 2017 was announced by the president on Twitter as his administration prepares for an unprecedented meeting with the leader of North Korea.

Mr Trump tapped the CIA's deputy director, Gina Haspel, to replace Mr Pompeo at the intelligence agency.

Mr Tillerson's departure caps months of friction between the Republican president and the 65-year-old former Exxon Mobil chief executive.

Today, Mr Tillerson did not address reports that he had only learned of his sacking when he read about it in Trump's tweet, but said he had since spoken to the president by telephone.

The tensions peaked last Autumn amid reports Mr Tillerson had called Mr Trump a "moron" and considered resigning.

"We've been talking about this for a long time," Mr Trump said on the White House lawn today. "We got along actually quite well but we disagreed on things."

He cited the Iran nuclear deal as an example of disagreements with Mr Tillerson and said he and Mr Pompeo have "a similar thought process."

Senior State Department officials said Mr Tillerson was unaware of why Mr Trump pushed him out and that he had intended to stay in the job.

Calling your boss a "moron" is never a good sign.

It was reported last year that Rex Tillerson had used that word to describe Donald Trump, a clear indication of the difficult relationship the two men had.

Those divisions became even more evident in recent days. Just yesterday, Rex Tillerson issued a statement on the nerve agent attack in Britain that appeared to be at odds with the White House.

Last week, it looked like Mr Tillerson was unaware of the President's plan to meet North Korean leader Kim Jong-un.

This morning's tweet from Donald Trump confirming the sacking caught many people by surprise.

But given the worsening relations between the President and his top diplomat, perhaps the real surprise here is that the announcement wasn't made sooner.

Tillerson blamed Russia for nerve agent attack in UK

Yesterday, Mr Tillerson blamed Russia for the poisonings in England of a former Russian double agent and his daughter.

Earlier, White House press secretary Sarah Sanders had refrained from saying Moscow was responsible.

A senior White House official said Mr Trump asked Tillerson to step down on Friday but did not want to make it public while he was on a trip to Africa.

Mr Trump's Twitter announcement came only a few hours after Mr Tillerson landed in Washington after the trip, which had been cut short.

The official said Mr Trump works well with Mr Pompeo, a former Republican congressman from Kansas, and wanted him in place before the president's planned talks with North Korean leader Kim Jong-un and trade negotiations.

Mr Tillerson had no diplomatic or political experience before becoming secretary of state.

He appeared out of the loop last week when Mr Trump announced he would meet with Mr Kim.

Tillerson joins long list of senior officials out of the White House

Mr Tillerson joined a long list of senior officials who have either resigned or been fired since Mr 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, economic adviser Gary Cohn and press secretary Sean Spicer.

Mr Trump publicly undercut Mr Tillerson's diplomatic initiatives numerous times and Mr Tillerson appeared out of the loop this month when Mr Trump announced plans to meet with North Korea's Kim.

Senior Trump administration officials also made it known in November that the White House had drafted a plan to replace Mr Tillerson.

In December Mr Tillerson had offered to begin direct talks with North Korea without pre-conditions, backing away from a key US demand that Pyongyang must first accept that any negotiations would be about giving up its nuclear arsenal.

But the White House distanced itself from those remarks, and a few days later, Mr Tillerson himself backed off.

Several months earlier in Beijing, Mr Tillerson told reporters the United States was directly communicating with North Korea but that Pyongyang had shown no interest in dialogue.

Mr Trump contradicted Tillerson's efforts a day later.

"I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man," Mr Trump wrote on Twitter, using a pejorative nickname for Kim.

"Save your energy Rex, we'll do what has to be done!" Mr Trump added.

Early in 2018, Trump said on Twitter that Mr Tillerson was "wasting his time" trying to negotiate with North Korea.Tillerson had joined Defense Secretary Jim Mattis in pressing a sceptical Mr Trump to stick with the agreement with Iran and other world powers over Tehran's nuclear ambitions and he has taken a more hawkish view than Mr Trump on Russia.

In June, Mr Tillerson urged Saudi Arabia and other Gulf Arab states to ease a blockade against Qatar, which they accuse of supporting Islamist militants and Iran. Qatar calls these charges baseless.

Hours later, however, Mr Trump accused Qatar of being a "high level" sponsor of terrorism, appearing to side firmly with Saudi Arabia, the United Arab Emirates, Bahrain and Egypt in the dispute and to undermine Mr Tillerson's effort to adopt a more even-handed approach to broker a solution.

Mr Tillerson also won few plaudits from career diplomats for his performance at the State Department.

His tenure coincided with the departure of dozens of veteran foreign policy hands, and most of the senior policy-making positions remain unfilled.