U.S. President Donald Trump said on Monday he had "terminated" Defense Secretary Mark Esper, replacing him in an acting capacity with Christopher Miller.

Trump had split with Esper over a range of issues and was particularly angered by his public opposition to Trump's threats to use active duty military forces this summer to suppress street protests over racial injustice after police killed George Floyd in Minneapolis.

Democrats reacted with alarm, saying Trump's move sent a dangerous message to America's adversaries and dimmed hopes for an orderly transition as the projected winner of the U.S. presidential election Joe Biden prepares to take office.

"The abrupt firing of Secretary Esper is disturbing evidence that President Trump is intent on using his final days in office to sow chaos in our American Democracy and around the world," said House of Representatives Speaker Nancy Pelosi.

Representative Adam Smith, the Democrat who leads the House Armed Services Committee, condemned Trump's decision as "childish" and "reckless."

Trump said on Twitter that Christopher Miller, director of the National Counterterrorism Center, was taking over as acting secretary of defense.

"Mark Esper has been terminated," Trump wrote in a tweet, adding that Miller would be acting secretary "effective immediately."

U.S. defense officials, speaking on condition of anonymity, said White House Chief of Staff Mark Meadows called Esper minutes ahead of time to alert him that Trump would be firing him through Twitter.

As Trump put into motion a quick, unceremonious exit for Esper, Miller arrived at the Pentagon building just an hour or so after Trump's announcement – before the Pentagon itself had even issued a statement acknowledging Esper's dismissal.

It was unclear if Esper was still in the building at the time Miller arrived.

Sources said Esper had long been preparing for his resignation or dismissal following last week's election, particularly if Trump were to win a second term in office. The fact that he dismissed Esper even after losing the election, however, was not a given.

In a letter to the Defense Department issued early Monday evening, Esper said he was stepping aside aware that "there is much more we could accomplish."

Esper commended the military for remaining "apolitical," a refrain that he often used and which Trump's opponents saw as implicit criticism of the president's attempts to portray the military as his constituency amid defense budget hikes.

(With input from Reuters)