Washington: In an extraordinary rebuke, former defence secretary Jim Mattis on Wednesday denounced President Donald Trump's heavy-handed use of military force to quell protests and said his former boss was setting up a "false conflict" between the military and civilian society.

"I have watched this week's unfolding events, angry and appalled," Mattis wrote.

The President fired back via Twitter later Wednesday night, saying: "Probably the only thing Barack Obama & I have in common is that we both had the honor of firing Jim Mattis, the world’s most overrated General. I asked for his letter of resignation, & felt great about it. His nickname was 'Chaos', which I didn’t like, & changed to 'Mad Dog'...

Trump said he had given Mattis "a new life, things to do, and battles to win," when he hired him, but but "I didn’t like his “leadership” style or much else about him, and many others agree. Glad he is gone!"