President Donald Trump, in a surprise Twitter message early on Wednesday, said he will nominate former assistant US attorney general Christopher Wray to lead the FBI, whose former chief was fired by Trump less than a month ago.

James Comey, dismissed as Federal Bureau of Investigation director on May 9, is slated to testify before a Senate panel on Thursday about investigations of possible ties between Russian meddling in the 2016 US elections and the Trump campaign.

Wray, now in private practice at law firm King & Spalding, worked from 2003 to 2005 at the Justice Department under former Republican President George W Bush.

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

The US Senate must evaluate and vote on any nomination by Trump of a new FBI leader.

His announcement comes as the Senate Intelligence Committee prepares to hear from top US intelligence officials and Comey regarding the FBI's probe into Russian involvement in the election and fallout from Comey's firing.

On Wednesday, lawmakers will hear from Deputy Attorney General Rod Rosenstein, the No. 2 official at the Justice Department who signed a letter recommending Comey's dismissal.

US Senator Mark Warner, the top Democrat on the intelligence committee, said in a series of television interviews that Trump's announcement seemed timed to distract from the two days of potentially explosive hearings.

"It appears the president is trying to change the topic," Warner told MSNBC.

The president met last week with candidates for the FBI director post, including Wray, according to White House spokesman Sean Spicer.

Wray works for King & Spalding's Washington and Atlanta offices, handling white-collar criminal and regulatory enforcement cases, according to the firm.

While there, he represented Chris Christie in the Bridgegate scandal, in which two of the New Jersey governor's aides were convicted. Christie, who became a close adviser to Trump during the campaign and whose name was floated as a possible Comey replacement, was never charged.

At the Justice Department, Wray worked on corporate fraud scandals and cases involving US financial markets, according to his biography on the law firm's website.

He represented the government in its fraud case against Enron Corp, the collapsed energy company.