Trump, who announced the decision on Twitter on Monday, named Mr Christopher Miller, Director of the National Counterterrorism Centre, as Acting Secretary of Defence.

“Mark Esper has been terminated. I would like to thank him for his service.

“I am pleased to announce that Christopher C. Miller, the highly respected Director of the National Counterterrorism Center (unanimously confirmed by the Senate), will be Acting Secretary of Defense, effective immediately.

“Chris will do a GREAT job!”, the tweet reads.

The move comes amid the president’s refusal to accept the results of the Nov. 3 presidential election won by former Vice President Joe Biden.

It also comes five months after Esper opposed his decision to deploy the military to quell the Black Lives Matter protests across the country.

“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.

“We are not in one of those situations now. I do not support invoking the Insurrection Act,” Esper has said in June.

According to ABC News, Trump was unhappy with Esper for opposing his attempt to invoke the Insurrection Act.

Esper has also reportedly differed with Trump’s dismissive attitude toward the North Atlantic Treaty Organisation (NATO).

Reports say the former defence secretary had prepared his resignation letter, and was “strongly considering resigning ahead of a possible firing”.

Miller assumed duty as the seventh Director of the National Counterterrorism Centre in August, after serving as deputy assistant secretary of defense for special operations and combating terrorism.(NAN)