WASHINGTON - U.S. President Donald Trump assailed his former White House chief of staff, John Kelly, on Thursday after Kelly defended the National Security Council aide ousted by Trump after he testified against the president during the impeachment investigation.

Kelly, a retired Marine Corps general who served as Trump's top White House aide for 17 months until early last year, voiced support for Army Lt. Col. Alexander Vindman at a university speech Wednesday night in New Jersey, while also raising objections to several Trump policies and actions.

"When I terminated John Kelly, which I couldn't do fast enough, he knew full well that he was way over his head," Trump contended in a Twitter comment.

"Being Chief of Staff just wasn't for him," Trump claimed. "He came in with a bang, went out with a whimper, but like so many X's, he misses the action & just can't keep his mouth shut, which he actually has a military and legal obligation to do. His incredible wife, Karen, who I have a lot of respect for, once pulled me aside & said strongly that 'John respects you greatly. When we are no longer here, he will only speak well of you.' Wrong!"

Kelly said that Vindman, as a military officer, "did exactly what we teach them to do from cradle to grave. He went and told his boss what he just heard."

Vindman expressed concern to his superiors last year after Trump asked Ukrainian President Volodymyr Zelenskiy to "do us a favor," to launch an investigation of one of Trump's chief 2020 Democratic rivals, former Vice President Joe Biden, and his son Hunter Biden's work for a Ukrainian natural gas company.

Then, Vindman late last year was a key witness against Trump as Democrats in the House of Representatives moved to impeach Trump, although the president was acquitted last week after a Senate trial.

"We teach them: Don't follow an illegal order," Kelly said of Vindman. "And if you're ever given one, you'll raise it to whoever gives it to you that this is an illegal order, and then tell your boss."

Vindman, along with his twin brother, Yevgeny (Eugene) Vindman, another Army lieutenant colonel working at the National Security Council, were both escorted from the White House last Friday and returned to their previous postings at the Defense Department. Critics have accused Trump of being vindictive in ousting Alexander Vindman, but the president called him "very insubordinate."

In the university speech, Kelly said he had had a number of disagreements with Trump.

Kelly defended the national news media, a frequent target of Trump's ire; questioned the worth of Trump's two summits with North Korean leader Kim Jong Un to try to get him to abandon his nuclear arsenal, and attacked Trump's pardon of a naval commando who had been convicted of war crimes.

Kelly, who before overseeing the White House staff was head of the U.S. Department of Homeland Security, also took issue with Trump's frequent disparagement of migrants looking to move to the United States.