Powell, who served in three different GOP administrations, also called Trump an "international pariah" and part of a "racist" birther movement designed to discredit President Barack Obama.
In a June 17 email exchange between Powell and journalist Emily Miller, who is a former aide to Powell, the retired four-star U.S. Army general reportedly said Trump "is in the process of destroying himself, no need for Dems to attack him. [Speaker of the House] Paul Ryan is calibrating his position again."
When asked about the email exchange, Powell told BuzzFeed News, "I have no further comment. I'm not denying it."
Powell was also upset with the national news media for its coverage of the candidate, telling Miller "To go on and call him an idiot just emboldens him."
Powell rejected calls to make his own run for president, but surprised many fellow Republicans in 2008 with his support for then-Sen. Obama.