Former Pentagon chief Jim Mattis issued a stinging rebuke of his erstwhile boss Donald Trump on Wednesday, accusing the president of trying to "divide" America and failing to provide "mature leadership" as the country reels from days of protests.

Mattis, who resigned in December 2018 over Trump's ordering of a full troop withdrawal from Syria, also voiced support for the demonstrators whose anti-racism rallies have roiled the country.

"Donald Trump is the first president in my lifetime who does not try to unite the American people — does not even pretend to try," Mattis wrote in a blistering statement posted online by The Atlantic.

"Instead, he tries to divide us," added the retired Marine general, who had previously argued it would be inappropriate for him to criticise a sitting president.

"We are witnessing the consequences of three years without mature leadership," he stated.

Mattis described himself as "angry and appalled" after witnessing events of the last week, which saw Trump threaten a military crackdown on American citizens as nationwide protests turned violent in some cities.

The fury was ignited by the May 25 killing of George Floyd, a black man who suffocated beneath the knee of a white police officer, and whose agonising death was filmed by bystanders.

The demonstrations have mostly been peaceful, but some have degenerated into violence and looting as night falls.

Mattis wrote that the protesters' call for equal justice was a "wholesome and unifying demand."

And he slammed the decision to use force to clear peaceful protesters from near the White House on Monday to allow Trump to pose for photographs at a nearby damaged church, calling it an "abuse of executive authority."

The photo op has become a lightning rod for criticism of Trump's handling of the crisis, with religious leaders, politicians, and onlookers around the country expressing outrage.

"When I joined the military, some 50 years ago, I swore an oath to support and defend the Constitution," Mattis stated.

"Never did I dream that troops taking that same oath would be ordered under any circumstance to violate the Constitutional rights of their fellow citizens — much less to provide a bizarre photo op for the elected commander-in-chief, with military leadership standing alongside."

Trump dismissed Mattis with a tweet, rehashing his claim that he "essentially" fired his Pentagon chief.

"Probably the only thing Barack Obama and I have in common is that we both had the honor of firing Jim Mattis, the world's most overrated General," the president wrote.

Mattis was head of US Central Command when Obama fired him in 2013 over his hawkish views on Iran.

'We can unite without him'

For months after Mattis resigned, he refused to criticize Trump publicly, insisting the military must remain apolitical.

Wednesday's statement appeared to signaled he no longer felt bound by that sentiment, as he called for solidarity — with or without the president.

"We can unite without him, drawing on the strengths inherent in our civil society," Mattis wrote.

Retired Marine Corps General John Allen echoed Mattis' criticism of Trump after his speech threatening to deploy the US military against American citizens.

"To even the casual observer, Monday was awful for the United States and its democracy," the former commander of US and NATO forces in Afghanistan wrote in Foreign Policy.

"The president's speech was calculated to project his abject and arbitrary power, but he failed to project any of the higher emotions or leadership desperately needed in every quarter of this nation during this dire moment."

Allen, president of the Brookings Institution, also took aim at the president's church photo-op.

"Donald Trump isn't religious, has no need of religion, and doesn't care about the devout, except insofar as they serve his political needs," he wrote.