President Donald Trump's former chief of staff John Kelly said he believed the former national security adviser John Bolton's allegations concerning Trump's Ukraine pressure campaign.

Bolton reportedly wrote in his forthcoming book that the president told him last year that he would withhold military aid to Ukraine until the Ukrainian president initiated investigations into Trump's political rivals.

"If John Bolton says that in the book, I believe John Bolton," Kelly, a retired general who also served as Trump's secretary of homeland security, told an audience in Florida on Monday night.

Kelly described Bolton as honest and said he supported the Senate calling witnesses in its impeachment trial — something the president and Republican leaders have staunchly opposed.

Visit Business Insider's homepage for more stories.

President Donald Trump's former chief of staff John Kelly said he believed the former national security adviser John Bolton's allegations concerning Trump's Ukraine pressure campaign.

The New York Times reported on Sunday that Bolton wrote in his forthcoming book that the president told him last year that he would withhold military aid to Ukraine until the Ukrainian president acceded to his demands for investigations into his political rivals.

Bolton's allegation contradicts Trump's repeated claim that he didn't leverage the military aid to pressure Ukraine to investigate former Vice President Joe Biden and other Democratic opponents.

"If John Bolton says that in the book, I believe John Bolton," Kelly, a retired general who also served as Trump's secretary of homeland security, told an audience in Florida on Monday night, the Sarasota Herald-Tribune reported.

Kelly described Bolton as honest and said he supported the Senate calling witnesses in its impeachment trial — something the president and Republican leaders have staunchly opposed.

Kelly said, according to the Herald-Tribune, that "every single time" he was with his former White House colleague, "he always gave the president the unvarnished truth."

Kelly's comments were an astonishing rebuke of the president he served as a close West Wing adviser to for 18 months, from July 2017 to January 2019.

Trump has denied Bolton's claims and argued that he is simply trying to sell his book.

"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," the president tweeted on Sunday. "If John Bolton said this, it was only to sell a book."