US President Donald Trump has fired his defense secretary, Mark Esper, amid political chaos gripping his administration after his refusal to concede the election defeat.

Trump said in a tweet on Monday that Esper had been “terminated” and that Christopher Miller, director of the National Counterterrorism Center, would “immediately” assume the post.

“Mark Esper has been terminated,” Trump said on Twitter. “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.”