US President Donald Trump has announced Christopher Wray as his pick for FBI director.

Trump's early morning two-sentence tweet nominating Wray, a lawyer, came one day before the FBI director that Trump fired last month, James Comey, was to testify in public on Capitol Hill for the first time since his dismissal.

Trump called Wray "a man of impeccable credentials" and offered no more information about the selection, ending the tweet by saying, "Details to follow."

Wray served in a leadership role in the George W. Bush Justice Department, rising to head the criminal division and overseeing investigations into corporate fraud, during the time when Comey was deputy attorney general.

Wray took charge of a task force of prosecutors and FBI agents created to investigate the Enron scandal.

Comey, during his appearance before the Senate intelligence committee, is expected to describe his encounters with Trump in the weeks before his firing on May 9. He could offer new details regarding discussions with Trump about the federal investigation into Russia's election meddling.

Wray works in private practice for the King & Spalding law firm. He worked as a federal prosecutor in Atlanta, Georgia, from 1997-2001, and was associate deputy attorney general from 2001-03.

He graduated with honours from Yale University in 1989 and in 1992 from Yale Law School.