US President Donald Trump said on Wednesday that John Bolton, dismissed a day earlier as national security adviser, had been a "disaster" on North Korea policy, "out of line" on Venezuela, and did not get along with important administration officials.
Mr Trump said Mr Bolton had made mistakes, including offending North Korea's leader Kim Jong-un by demanding that he follow a "Libyan model" and hand over all his nuclear weapons.
"We were set back very badly when John Bolton talked about the Libyan model. What a disaster," the president said.
"He's using that to make a deal with North Korea? And I don't blame Kim Jong-un for what he said after that, and he wanted nothing to do with John Bolton.
"And that's not a question of being tough. That's a question of being not smart to say something like that."
Mr Trump also said he disagreed with Bolton on Venezuela but offered no specifics.
"I thought he was way out of line and I think I've proven to be right," he said.
Mr Trump said Mr Bolton, with his abrasive, hardline approach, "wasn't getting along with people in the administration that I consider very important".
"John wasn't in line with what we were doing," he said.
Mr Trump said he got along with Mr Bolton and hoped they parted on good terms.
"Maybe we have and maybe we haven't," he said. "I have to run the country the way we're running the country."
Mr Trump had been growing more impatient with the failure to oust socialist Venezuelan President Nicolas Maduro through a campaign of sanctions and diplomacy, in which Mr Bolton was a driving force.
He was also a chief architect of the Trump administration's hardline policy on Iran.
Mr Bolton had opposed moves such as easing sanctions on Iran to secure a meeting with President Hassan Rouhani at this month's UN General Assembly, something at which Mr Trump has hinted.
"We'll see what happens," the US president said.
North Korea has denounced Mr Bolton as a "war maniac" and "human scum".
Last year, it threatened to call off a first summit between Mr Kim and Mr Trump after Mr Bolton suggested the Libyan model of unilateral disarmament.
In the past, he proposed using military force to overthrow the country's ruling dynasty.
Mr Trump's efforts to engage with North Korea nearly fell apart altogether in February after he followed Mr Bolton's advice at a second summit in Hanoi.
He handed MR Kim a piece of paper that called for the transfer of Pyongyang's nuclear weapons and bomb fuel to the Us.
Mr Trump announced he had fired Bolton a day after North Korea signalled a new willingness to resume stalled talks on ridding the Korean Peninsula of nuclear weapons.
But it then proceeded with its latest missile test launches.