WASHINGTON - US President Barack Obama plans to officially nominate former Deputy Attorney General James Comey to replace Robert Mueller as next director of the FBI, US media quoted White House officials as saying on Thursday.

Obama will make the announcement at a White House ceremony on Friday afternoon. Comey, the No 2 official in former President George W. Bush's Justice Department, had been seen as his top choice for the next FBI director.

A White House official said Comey has served as a prosecutor and national security professional in more than two decades, and demonstrated "unwavering toughness, integrity and principle," reported the Hill, a congressional newspaper.

If nominated and approved by Congress, Comey will replace current Director Mueller, whose term had been extended at Obama's request by two years and now scheduled to expire in September this year.

Outgoing Director Mueller has served as the agency's head for almost 12 years, starting just days before the September 11 terror attacks in 2001.

Mueller is leaving as the agency faces a number of questions recently, from whether more could have been done to prevent the Boston bombings to what role it has played in the recently revealed phone and internet surveillance programs.

Testifying before a Senate panel on Wednesday, Mueller said, " As illustrated by the recent attacks in Boston, the terrorist threat against the United States must remain our top priority."

He stressed that the country is facing "a continuing threat from homegrown violent extremists" while foreign terrorists still seek to strike Americans at home and abroad.

"And just as our national security and criminal threats constantly evolve, so too must we, the FBI, evolve to counter these threats, even during a time of constrained budgets," said Mueller.

Last week, Mueller defended two classified phone and internet surveillance programs, which sparked controversy recently, and vowed to hold the leaker responsible for the disclosure.

He told lawmakers during a hearing that the disclosures about these secret surveillance programs have caused "significant harm to our nation and to our safety."

He even suggested that if the surveillance programs had been in place before the September 11, 2001 terrorist attacks, they might have helped to yield evidence of connections of the participants and derail the plan.

According to the Guardian and the Washington Post reports last Thursday, the National Security Agency and the FBI had been secretly tapping directly into the central servers of nine US internet companies, extracting audio, video, photographs, e-mails, documents and connection logs that enable analysts to track a person's movements and contacts over time.