The nomination of former US ambassador to the UN John Bolton to become President Donald Trump's newest national security adviser received\u00a0no shortage criticism, and on Monday, former President Jimmy Carter weighed in, calling it Trump's "worst mistake."\r\n"I have been concerned at some of the things he's decided," Carter\u00a0said\u00a0to CBS "This Morning" co-host Norah O'Donnell. "I think his last choice for national security adviser was very ill-advised."\r\n"I think John Bolton has been the worst mistake he's made," Carter continued.\r\nBolton was selected to become national security adviser, the third in Trump's tenure, after H.R. McMaster\u00a0resigned on Thursday.\r\nCarter has\u00a0offered to help\u00a0the White House on the US's latest developments with North Korea and been briefed by White House officials. The former president previously made a trip to North Korea in 1994, where he secured the release of an American citizen.\r\n\u00a0\r\nCarter said he believes Trump "reacted quite well" to North Korean leader Kim Jong-un's interest in holding talks, but said Trump's rhetoric toward the regime has been largely unhelpful.\r\n"I have had some criticisms of some of the public statements that \u2014 that President Trump has made about \u2014 fire and brimstone and that \u2014 sort of thing being utilized," Carter said, apparently referring to Trump's August 2017 threat that the North Korean regime would "be met with\u00a0fire and fury like the world has never seen," and\u00a0suggesting\u00a0Kim Jong-un was "short and fat."