President Donald Trump announced Wednesday that he is tapping former Justice Department official Christopher Wray to serve as his new FBI director, on the eve of critical testimony by his ousted predecessor.

Wray, a partner at King & Spalding law firm in Washington and Atlanta, served as assistant attorney general in charge of the Justice Department's Criminal Division from 2003 to 2005, working closely with the FBI.

"I will be nominating Christopher A. Wray, a man of impeccable credentials, to be the new Director of the FBI. Details to follow," Trump wrote on Twitter.

At King & Spalding, the Yale Law School graduate chairs a unit representing entities and individuals in white collar criminal and regulatory enforcement issues, civil litigation and internal corporate investigations, according to the law firm's website.

At the Justice Department, he helped handle corporate fraud scandals, served on president George W. Bush's Corporate Fraud Task Force and oversaw major fraud investigations including that of energy giant Enron.

He also helped coordinate the agency's response to the September 11, 2001 attacks.

The announcement came just one day before sacked FBI director James Comey gives highly-anticipated testimony on Russia's interference in the 2016 US elections and possible collusion with Trump's campaign.

Comey is expected to dispute Trump's claim that the then-FBI chief told him multiple times that he was not under investigation, CNN reported, citing sources familiar with Comey's thinking.

That potential bombshell testimony -- in which Comey also may address whether Trump urged him to halt or ease up on an investigation into former national security advisor Michael Flynn and his ties to Russia -- takes place before the Senate Intelligence Committee.

Adding to the drama, a top secret NSA report leaked this week examines repeated attempts by hackers from Russian military intelligence to break into US voting systems before last year's presidential election.

Keen to crack down on leaks, the Trump administration quickly announced the arrest of 25-year-old intelligence contractor Reality Winner on charges she violated the espionage act.

No definitive evidence of collusion between Trump's campaign and Russia has yet come to light.

But the allegations have drawn comparison to the 1970s Watergate scandal that brought down president Richard Nixon.

(AFP)