"I think they realize that he is way, way out of his depth," Brennan told MSNBC host Chris Hayes, "that he is incompetent, but also that he is somewhat delusional as far as how he views the world and his inability to grasp the reality of the geo-strategic situation around the globe today."
Brennan's comments came as Trump met with heads of leading, industrialized nations at the G-7 summit.
The former CIA chief, who now serves as a senior analyst for NBC News and MSNBC, said that the world was trying to "handle Trump the best they can." He went on to say he missed being in the intelligence field because he would "love" to find out what foreign leaders say about Trump behind the scenes.
Trump received criticism for supporting the idea that Russia should rejoin the G-7. The nation originally left the organization amid controversy surrounding its 2014 decision to annex Crimea.
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"Donald Trump still does not grasp the seriousness of what it is that [Vladimir] Putin has done," Brennan said, "not only to Ukraine but also the fact that he interfered in our election, continues to support the genocide of President Assad in Syria — all of these transgressions that have come from the Russians."
Brennan argued that Trump should be "coming down hard" on the Russian leader rather than giving him the "benefit of the doubt."
Brennan has a history of attacking Trump during television appearances. In April, for example, Brennan derided what he called "sociopathic ramblings" from the president.
"I don't think it's surprising at all that we continue to hear the sociopathic ramblings of Mr. Trump claiming that there was this effort to try to prevent him from being elected or to unseat him," he said.
That followed remarks in which Trump criticized Brennan and other former high-ranking officials in intelligence and law enforcement.