Washington (CNN) Former White House chief of staff John Kelly says he believes John Bolton's allegation that President Donald Trump told the former national security adviser that US security aid to Ukraine was conditioned on an investigation of the President's political rivals, adding that Bolton should be heard from.

"If John Bolton says that in the book I believe John Bolton," Kelly said Monday night when asked about the leaked draft manuscript during remarks at the Ringling College Library Association Town Hall lecture series, according to the Herald Tribune of Sarasota, Florida.

Kelly said Bolton "always gave the president the unvarnished truth" and is a "man of integrity and great character."

"I mean, half of Americans think this process is purely political and shouldn't be happening, but since it is happening, the majority of Americans would like to hear the whole story," Kelly said.

"So I think if there are people that could contribute to this, either innocence or guilt ... I think they should be heard. I think some of the conversations seem to me to be very inappropriate but I wasn't there. But there are people that were there that ought to be heard from."

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