Asked if he thinks Russians "have something on Donald Trump" Comey said: "I think it's possible", CNBC reported.
"It is stunning and I wish I wasn't saying it, but it's the truth. I cannot say that (Trump is not subject to Russian blackmail).
"It always struck me and still strikes me as unlikely, and I would have been able to say with high confidence about any other president I dealt with, but I can't. It's possible."
The White House did not immediately respond to CNBC's request for comment about the Comey interview.
Comey also told ABC there was "certainly some evidence of obstruction of justice" when Trump asked him in 2017 whether he could see his way to "letting Flynn go," referring to a criminal probe of fired National Security Advisor Michael Flynn.
"If he didn't know he was doing something improper, why did he kick out the attorney general... and the leaders of the intelligence community?" Comey asked referring to Trump's actions. "... Doesn't he know the nature of the request?"
Comey's interview comes in advance of publication of his new book, "A Higher Loyalty: Truth, Lies, and Leadership" on Tuesday.
Mueller is investigating Russian interference in the 2016 presidential election, and possible collusion by Trump campaign officials in that meddling.
Trump has denied any such collusion. He also fumed about Mueller, and considered firing the special counsel, along with Attorney General Jeff Sessions, whose recusal from the Russia probe led to Mueller's appointment by Deputy AG Rod Rosenstein.
Comey said he does not know "what's behind" Trump's reluctance to criticise Russian President Vladimir Putin in public.
"That mystified me even after President Trump became president, 'cause I discovered that he wouldn't criticize him even in private," Comey said.
Asked whether Trump was unfit to be president and should be impeached, Comey said people need to stand up and go to the voting booth and vote their values.
On Sunday, Trump unloaded a series of nasty tweets about Comey, and called him "slippery," and a "slime ball" and predicted that he would be remembered "as the Worst FBI Director in history, by far!"
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)