WASHINGTON: President Donald Trump on Monday announced by tweet that he had fired his defense secretary Mark Esper, further destabilizing a government already navigating Trump's refusal to concede election defeat to Democrat Joe Biden.

"Mark Esper has been terminated. I would like to thank him for his service," Trump said on Twitter, announcing his replacement by Christopher Miller, the current head of the National Counterterrorism Center.

The firing of Esper, who had clashed with Trump over his suggestion of using military personnel to quash civic unrest, comes a week after the US presidential election.

Trump, who is pursuing so far flimsy claims of election fraud in the courts, has only until January 20 before he has to leave office and Biden takes over.