President Donald Trump's respected former chief of staff let rip against the "illegal" scheme in Ukraine that led to impeachment, The Atlantic reported on Thursday.

In an unusually blunt speech late on Wednesday, retired Marine Corps general John Kelly also criticised Trump's policies on North Korea, immigration and intervention in the case of a special forces soldier accused of war crimes.

Kelly, who served in the White House from 2017-2019, sprang to the defence of national security advisor Alexander Vindman, who testified against Trump in his impeachment probe, only to be fired last Friday in apparent retaliation.

Vindman lodged an alert after hearing Trump make his now infamous phone call to new Ukrainian President Volodymyr Zelensky in July, demanding a probe into Democratic presidential opponent Joe Biden.

Vindman, an army officer, was reacting as he should any time on hearing "an illegal order", Kelly said in the speech at Drew University in New Jersey, The Atlantic reported.

"We teach them, 'Don't follow an illegal order. 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.'"

Kelly, who saw Trump up close during the heart of his first term, also criticised two of the president's highest profile initiatives.

He said that a dramatic attempt to secure a breakthrough over North Korea's nuclear arsenal by meeting twice with leader Kim Jong Un had failed.

"He will never give his nuclear weapons up," Kelly said of the North Korean leader. "I never did think Kim would do anything other than play us for a while, and he did that fairly effectively."

He also blasted Trump's portrayal of illegal immigrants as dangerous criminals, saying "in fact, they're overwhelmingly good people."

"They're not all rapists and they're not all murderers. And it's wrong to characterise them that way. I disagreed with the president a number of times," he said.

Trump's order for the Navy to reverse its expulsion of a Navy SEAL member, Eddie Gallagher, after he was convicted of posing for a photo with a dead prisoner in Iraq was "the wrong thing to do," Kelly said.

"Had I been there, I think I could have prevented it," he said.

Gallagher was acquitted of charges that he knifed the wounded, teenage prisoner to death before posing for the trophy photo.