Donald Trump’s former defence secretary James Mattis has condemned the US President’s response to mass protests across the US, calling the President a threat to the Constitution.

General Mattis was one of Mr Trump’s most high-profile appointments.

He resigned as US secretary of defence in December 2018 and has largely remained silent about Mr Trump since. However, on Wednesday (US time), he released an extraordinary statement, condemning the President’s aggressive response to protests that have rocked the US for nine days.

“Donald Trump is the first president in my lifetime who does not try to unite the American people – does not even pretend to try. Instead he tries to divide us,” General Mattis said.

“We are witnessing the consequences of three years of this deliberate effort. We are witnessing the consequences of three years without mature leadership.

“We can unite without him, drawing on the strengths inherent in our civil society. This will not be easy, as the past few days have shown, but we owe it to our fellow citizens; to past generations that bled to defend our promise; and to our children.”

National Guard troops roll towards the White House on June 1. Photo: Getty

General Mattis’s remarks follow more than a week of nationwide protests after the death of George Floyd in Minneapolis on May 25.

One of the officers, Derek Chauvin was captured on video pressing his knee into Mr Floyd’s neck for nearly nine minutes, before the African-American man died. He has been charged with second-degree murder, while three other officers involved in the arrest have been charged with aiding and abetting murder.

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On Wednesday, current US Defence Secretary Mark Esper pushed back against Mr Trump’s warning that he would use military force to clamp down on wild protests.

Mr Esper said he opposed the use of troops – despite Mr Trump’s warning to the states that he was willing to send soldiers to “dominate” their streets.

Mr Trump’s move would be legal, under the US’s 1807 Insurrection Act. But Mr Esper said the law should be invoked “only in the most urgent and dire of situations.”

“We are not in one of those situations now,” he said.

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However, Mr Esper abruptly overturned an earlier Pentagon decision to send home hundreds of soldiers from Washington, DC, region, amid growing tensions with the White House.

White House officials had indicated, even before Mr Esper’s comments, that Mr Trump was backing away from invoking the 1807 law. But they said Mr Trump was upset that Mr Esper’s statement conveyed “weakness.”

Press secretary Kayleigh McEnany said the President was still willing to deploy federal troops.

“If needed, he will use it,” she said. “But at this time he’s relying on surging the streets with National Guard. It’s worked with great effect.”

Mr Trump is also taking credit for deploying federal and other law enforcement officers to the US capital, saying it offered a model to states for stopping the violence accompanying some protests.

He said the massive show of force had helped to calm protests in Washington and other cities in recent days.

Mr Esper has also come under fire from critics. He was among the group that walked with Mr Trump from the White House on Monday for a presidential photo opportunity in front of St John’s Episcopal Church.

Mr Esper said that while he was aware they were heading to St John’s, he did not know what would happen there, and did not know that police had forcibly moved peaceful protesters out to clear the way for Mr Trump and his entourage.

-with AAP