(CNN) Former FBI Director James Comey said he believes "it's possible" that Russia has compromising information on President Donald Trump.

"I think it's possible. I don't know. These are more words I never thought I'd utter about a President of the United States, but it's possible," Comey told ABC News during a wide-ranging, five-hour long interview, a portion of which aired on Sunday evening, promoting his tell-all book "A Higher Loyalty," due out this week.

"I (would have) been able to say with high confidence about any other President I dealt with, but I can't. It's possible," Comey said.

Comey was fired by the President last year in the midst of the FBI's investigation into whether Trump's campaign associates colluded with Russia to influence the 2016 election.

When he was still FBI chief, Comey briefed Trump on the details of a dossier -- portions of which remain unconfirmed -- that was commissioned as opposition research during the 2016 election and compiled by a former British intelligence agent, Christopher Steele. The dossier contained salacious allegations about the President during a trip he took to Moscow in 2013.

Read More