The ousted inspector general of the United States intelligence community says he is "disappointed and saddened" that President Donald Trump fired him, but he also encouraged other inspectors general to continue to speak out when they are aware of wrongdoing.

Trump notified Congress late on Friday evening that he intended to fire Michael Atkinson, a pivotal figure in his impeachment last year, because he had lost confidence in him.

On Saturday, Trump made it clear that the move had been retaliatory, telling reporters that Atkinson was a "disgrace" and had done "a terrible job" because he had provided an anonymous whistleblower complaint to Congress - a move that was required by law.

Atkinson said in the statement, sent to reporters late on Sunday, that "it is hard not to think that the president's loss of confidence in me derives from my having faithfully discharged my legal obligations as an independent and impartial inspector general, and from my commitment to continue to do so".

Atkinson was required by law to notify Congress of the complaint, which was written by an anonymous intelligence official and detailed Trump's pressure on Ukraine to investigate Democrats. The inspector general had deemed it urgent and credible, meaning that he was required to share it with the House and Senate intelligence committees.

But the acting director of national intelligence at the time, Joseph Maguire, overruled him for several weeks.

After a firestorm sparked by media reports of the complaint, it was turned over and made public in September, and a congressional inquiry into the matter led to Trump's impeachment by the House in December. The Republican-led Senate acquittedTrump in February.

Trump has previously relieved of duty those with whom he disagreed in his administration [Al Drago/Reuters]

Atkinson said in the email that he was legally obligated to "ensure that whistle-blowers had an effective and authorised means to disclose urgent matters involving classified information to the congressional intelligence committees," and that such whistleblowers were protected against reprisal. Trump repeatedly called for the whistle-blower's name to be revealed.

Atkinson also directed his message to other inspectors general, saying that he knows they will "continue to do everything in their power" to continue to protect whistle-blowers.

"Please do not allow recent events to silence your voices," Atkinson wrote.

Atkinson's statement was sent to reporters by email on Sunday evening and was copied to Alan Boehm, the executive director of the Council of the Inspectors General on Integrity and Efficiency. Boehm confirmed the authenticity of the letter in a follow-up email exchange with The Associated Press news agency.

Intelligence shakeup

On Saturday, Trump questioned why Atkinson did not speak to him about the complaint, though the inspector general's role is to provide independent oversight.

"Never came in to see me, never requested to see me," Trump said. He added: "That man is a disgrace to IGs."

Atkinson's removal is part of a larger shakeup of the intelligence community under Trump, who has long been sceptical of intelligence officials and information. Atkinson is at least the seventh intelligence official to be fired, ousted or moved aside since last summer.

His removal came under immediate fire from Democrats and a handful of Republicans.

Senator Chuck Grassley, the Iowa Republican who leads the Finance Committee, said that Congress has been "crystal clear" that written reasons must be given when inspectors general are removed for a lack of confidence.

"More details are needed from the administration," Grassley said.

Maine Senator Susan Collins, a Republican member of the Senate Intelligence Committee, said she did not find Trump's reasoning in his Friday letter to be persuasive, and said Atkinson's removal "was not warranted".

Senate Intelligence Committee Chairman Richard Burr, a Republican from North Carolina, said an inspector general "must be allowed to conduct his or her work independent of internal or external pressure".

Still, there is little Congress can do in response, especially as legislators are scattered across the country and out of session due to the coronavirus pandemic.

Trump said in the letter to the intelligence committees that Atkinson would be removed from office in 30 days, the required amount of time he must wait after informing Congress.

He wrote that he would nominate an individual "who has my full confidence" at a later date.