President Donald Trump's pick to lead the FBI broke with his prospective boss over the investigation into his presidential campaign's ties to Russia, dismissing Trump's claim that probe lacks merit.

Christopher Wray told the Senate intelligence committee he does not consider Special Counsel Robert Mueller "to be on a witch hunt" following Trump's claim the probe amounts to a "hoax" and "the greatest Witch Hunt in political history".

"I am very committed to supporting Director Mueller in the special counsel investigation in whatever way is appropriate," he said.

Wray said any attempt to interfere with Mueller's investigation should be dealt with "sternly and appropriately".

Trump nominated Wray to lead the FBI in June, one month after he abruptly dismissed former chief James Comey from the post. Trump and his White House have offered vacillating explanations for Comey's dismissal.

One such explanation, that Comey lost the confidence of the bureau's rank and file, was flatly rejected by Acting Director Andrew McCabe who took over after Comey's ouster.

In hours-long testimony, Comey said Trump demanded a pledge of loyalty and asked him to drop an investigation into a former top official who resigned in disgrace.

Such requests do not square with the way the bureau's independent nature, Wray suggested.

"There is only one right way to do this job, and that is with strict independence," he said, adding that the bureau must be run "without fear, without favoritism, and certainly without regard to any partisan political influence".

Prior to being nominated, Wray served for nearly a decade as a Justice Department prosecutor. He entered private practice in 2005 and has worked as a lawyer in the past decade defending large corporations, according to The Associated Press.