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Former White House chief strategist Steve Bannon said Sunday the firing of FBI Director James Comey may have been the biggest mistake in "modern political history."

Bannon confirmed in an online-only segment of "60 Minutes" that he opposed President Donald Trump's decision to oust Comey.

The wide-ranging discussion, which covered the Russia investigation, the media, the Republican Party and North Korea, was Bannon's first major interview since leaving the White House last month.

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Asked by Charlie Rose whether Comey's firing was the worst mistake in political history, he said: "That would probably be too bombastic even for me, but maybe 'modern political history.'"

Bannon said institutions such as the Senate and House of Representatives can be changed "if the leadership is changed." But he also said the FBI, which he called "an institution," is different.

"I don't believe that the institutional logic of the FBI, and particularly in regards to an investigation, could possibly be changed by changing the head of it," Bannon said.

Steve Bannon Bill O'Leary / The Washington Post via Getty Images file

The ousted White House adviser also said that if Comey hadn't been fired, "we would not have the Mueller investigation," referring to special counsel Robert Mueller, who is heading the investigation into possible ties between Trump's election campaign and the Russian government.

Trump was heavily scrutinized for citing this investigation as a reason he dismissed Comey in May. Mueller was appointed as special counsel for the investigation shortly after that move.

White House Press Secretary Sarah Huckabee Sanders responded Monday, saying the administration has "been pretty clear" in its reasoning for ousting Comey, and subsequent events showed that "the president was right in firing" the FBI director.

"Since the director's firing, we've learned new information about his conduct that only provided further justification," Sanders said, listing "false testimony" and leaking information to reporters among them.

Asked if the president was disappointed in Bannon's interview, Sanders said she wasn't sure. She also said that as far as she's aware, the two men have only spoken once since Bannon left his West Wing post.