US President Donald Trump has announced by tweet that he had fired his Defense Secretary Mark Esper.
The move further destabilises a government already navigating Mr Trump's refusal to concede the presidential election to Democrat Joe Biden.
"Mark Esper has been terminated. I would like to thank him for his service," Mr Trump said on Twitter.
He announced his replacement as Christopher Miller, the current head of the National Counterterrorism Center.
The firing of Mr Esper, who had clashed with Mr Trump over his suggestion of using military personnel to quash civic unrest, comes a week after the US presidential election.
Mr Trump, who is pursuing so far flimsy claims of election fraud in the courts, has only until 20 January before he has to leave office and Mr Biden takes over.
The Pentagon had no immediate comment.
Mr Esper had long been preparing for the prospect of his resignation or dismissal following the election, particularly if Mr Trump were to win a second term in office, sources said.
Mr Esper angered Mr Trump particularly by opposing his threat to use active duty troops to suppress street protests over racial injustice in the US during the summer.
He also disagreed with Mr Trump's dismissive attitude toward the NATO alliance, sources said.
