WASHINGTON - US President Donald Trump stepped up his offensive Friday against Robert Mueller's investigation into Russian election meddling, declaring it "totally discredited" following the release of a watchdog report documenting failings by the FBI.

The long-awaited report faulted the FBI and its former director James Comey over the handling of the Hillary Clinton email probe in 2016 - and concluded that two agents working under Comey showed a "willingness to take official action to impact" Trump's election chances.

Trump seized on the findings of the Justice Department Inspector General, claiming the report provides evidence of deep bias against him at the FBI, and "exonerates" him from allegations of collusion with Moscow and obstruction of justice. "I did nothing wrong, there was no obstruction. The IG report yesterday went a long way to show that and I think that the Mueller investigation has been totally discredited," Trump told journalists at the White House.

Trump has repeatedly assailed the Russia probe as a politically-motivated "witch hunt."

Earlier, he retweeted Fox & Friends quoting a contributor saying: "Anything Mueller is doing with his investigation is tainted by the anti-Trump FBI agent." Trump also took aim at Comey - who he fired in May 2017 and whose reputation he has since sought to tarnish in the expectation he could be a witness against him in the encroaching Russia investigation. "The IG Report is a total disaster for Comey, his minions and sadly, the FBI. Comey will now officially go down as the worst leader, by far, in the history of the FBI. I did a great service to the people in firing him," Trump tweeted.

The internal report reviewed one of the most controversial chapters of the 2016 election battle between the Republican Trump and his Democratic rival Clinton. It found Comey "insubordinate" and guilty of a "serious error of judgment" in how he handled the probe. Even so, the Justice Department's Inspector General found no fault in the decision announced by Comey on July 5, 2016 that Clinton should not face prosecution for placing classified materials on her personal email server while she was secretary of state.

In the middle of the campaign, Comey announced Clinton would not be prosecuted - a statement that would have normally been handled by the attorney general and drew heavy criticism.

But 12 days before the November 8 election, Comey reopened the investigation, after new evidence surfaced - a move that may have contributed to her defeat. Then he closed it again after the evidence proved to be inconsequential.

"We found no evidence that the conclusions by the prosecutors were affected by bias or other improper considerations," the report said.

Trump has repeatedly said a biased Justice Department and FBI let Clinton off easily, and he made "lock her up" a chant in his election rallies.

He likewise claims the Russia probe is driven by investigators from the Justice Department and FBI who harbor entrenched biases against him.

Comey aside, the report left lingering suspicions of bias on the part of FBI investigators, citing text messages between two, Peter Strzok and Lisa Page, who worked on both the Clinton and Russia investigations and who were having an affair during that period.

During the 2016 campaign, Page asked Strzok: "(Trump's) not ever going to become president, right? Right?!"

"No. No he won't. We'll stop it," Strzok replied.

The report said that there was no evidence that either ever acted on their sentiments, but assailed both for "extremely poor judgment and a gross lack of professionalism."

The White House immediately seized upon that aspect of the report, saying it "reaffirms the president's suspicions about Comey's conduct and the political bias among some of the members of the FBI."