Former FBI deputy director Andrew McCabe, who has endured a year of withering attacks from President Donald Trump, was fired by the Justice Department Friday just days before he was to retire.

"Andrew McCabe FIRED, a great day for the hard working men and women of the FBI - A great day for Democracy," Trump tweeted soon after the firing.

"Sanctimonious [ex-FBI director] James Comey was his boss and made McCabe look like a choirboy. He knew all about the lies and corruption going on at the highest levels of the FBI!" Trump wrote.

McCabe pushed back hard at the decision, saying he was the victim of a "war" by the Trump administration against both the FBI, and the special counsel probing allegations of Russian election meddling.

Announcing McCabe's ouster, the Justice Department said an internal investigation found that he had made unauthorized disclosures to the media, and had not been fully honest "on multiple occasions" with the department's inspector general.

Details of the inspector general's probe were not made public, but it involved the FBI's handling of the 2016 investigation into Trump's election rival, Democrat Hillary Clinton.

Trump has repeatedly accused McCabe and Comey of having protected Clinton from prosecution over her misuse of a private email server while she was secretary of state, and over the actions of the private Clinton Foundation set up by her husband, former president Bill Clinton.

Trump was also publicly upset about McCabe's defense of Comey -- whom the president fired in May 2017 in frustration at the ongoing FBI probe into alleged collusion between his election campaign and Russia.

The firing came as the White House shows increasing frustration with the collusion probe, now in the hands of Special Counsel Robert Mueller -- himself a former FBI director -- and increasingly threatening the president's inner circle.

Mueller is notably examining whether Trump obstructed the investigation with his sacking of Comey, an episode that McCabe would be a crucial witness for.