A rare public rift has opened between United States President Donald Trump and senior military leaders over Trump's threats to use troops against protesters as the US braces for another day of unrest and mourning following the death of George Floyd.

Secretary of Defense Mark Esper broke publicly with Trump on Wednesday in an appearance at the Pentagon and said active-duty military troops should not be used to quell the protests. Other military leaders soon followed.

"The option to use active-duty forces in a law enforcement role should only be used as a matter of last resort and only in the most urgent and dire situations," Esper said.

General Mark Milley, the top US commander, later the same day issued a memo to military leaders reminding them of their oaths to protect the US Constitution and the "right to freedom of speech and peaceful assembly".

The rare disagreement between a president and his generals comes at a time when Trump is facing plummeting public support for his handling of the simultaneous crises of the coronavirus pandemic, crushing unemployment and mayhem in the streets. Trump met with his top campaign advisers at the White House on Thursday after polls released on Wednesday evening suggested his prospects for re-election are tumbling, the Reuters news service reported.

Trump's aggressive use of federal law enforcement in Washington, DC - and his threats to call in combat troops against protesters in cities across the nation - have alarmed even some Republican leaders and politicians who had been supporters of the president.

Trump's former Trump defence secretary, General James Mattis, chimed in with a strongly worded statement criticising Trump directly for his divisive rhetoric during the protests.

"Militarizing our response, as we witnessed in Washington, DC, sets up a conflict - a false conflict between the military and civilian society," Mattis said.

Senator Lisa Murkowski, a Republican, told reporters at the US Capitol on Thursday that Mattis's public letter rang true for her and she now is unsure whether to support Trump's re-election.

"I thought General Mattis's words were true and honest and necessary and overdue and I have been struggling for the right words," Murkowski said. "Perhaps we're getting to the point where we can be more honest with the concerns that we might hold internally and have the courage of our own convictions to speak up."

In remarks to the nation on June 1, Trump positioned himself as "the president of law and order" and promised to dispatch "thousands and thousands of heavily armed soldiers, military personnel and law enforcement officers" to cities across the US.

Trump sparked outrage after the speech when federal officers forcibly cleared peaceful protesters from a public park near the White House so he could walk to a nearby church for a photo opportunity.

Retired General Martin Dempsey, a former top US commander, criticised Trump in a radio interview set to air on Friday.

"The idea that the military would be called in to suppress what for the most part were peaceful protests" is "very dangerous", Dempsey said.

Irked by Esper's comments, Trump called the defence secretary to a meeting at the White House late on Wednesday and confronted him about their disagreement, Bloomberg reported.

The president questioned senior White House aides about whether they thought Esper could continue to be effective in his job, according to the report.

Firing the secretary of defence would come with a political price, and while Trump may be unhappy with Esper undercutting his hardline stance on the potential use of troops, some Republicans in Congress sided with Esper instead of the president.

"He's doing a good job," Senator Lindsey Graham, a Republican ally of Trump, told reporters at the Capitol, according to Politico.

"There's no reason to let him go. That's all just a bunch of chatter. I have confidence in Secretary Esper," Graham said.

"He should be allowed to express his opinion and his advice should be heeded," said Senator Shelley Moore Capito, a Republican.

"I hope he would stay on. I like him. We've got enough vacancies," she said.

Kevin McCarthy, the top Republican in the House of Representatives, defended Trump and pointed out to reporters on a conference call on Thursday that the president's authority to use military force against violent protests has been used repeatedly by other presidents.

Since World War II, US presidents have used their authority to deploy the military to put down riots six times, most recently in Los Angeles in 1992.

"From the president's point of view, he is going to protect our citizens and protect the nation and he has the right to do it," McCarthy told reporters.

Democrats in Congress were less charitable.

"What is President Trump doing to this democracy? To the rule of law, the primacy of the Constitution?" Senate Minority Leader Chuck Schumer, a Democrat, said in remarks to the Senate. (Aljazeera)

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