Acting FBI Director Andrew McCabe testifies before the US Senate Select Committee on Intelligence on Capitol Hill in Washington, US May 11, 2017. [Photo/Agencies]

"We consider it to be a highly significant investigation," McCabe told the Senate Intelligence Committee in a hearing.

McCabe said "there has been no effort to impede our investigation to date," but promised to inform the Congress should the White House tries to interfere with the FBI investigation.

McCabe also refuted a White House claim Wednesday that fired FBI Director James Comey had lost the confidence of "rank and file" FBI staff, saying Comey "enjoyed broad support within the FBI and still does to this day."

"I hold Director Comey in the absolute highest regard. I have the highest respect for his consideration abilities and his integrity," McCabe said.

McCabe's hearing came after Comey was unexpectedly fired by US President Donald Trump on Tuesday evening. Trump said he made the decision based on advice from the deputy attorney general and attorney general that Comey was unfit to lead the FBI, but critics say the timing of the firing was suspicious.