US President Donald Trump has "terminated" Defense Secretary Mark Esper, appearing to use his final months in office after his election defeat to settle scores within his administration.
Trump split with Esper over a range of issues and was particularly angered by his opposition to the threat of using active military forces to suppress race protests after police killed George Floyd in Minneapolis.
Trump said on Twitter on Monday Christopher Miller, director of the National Counterterrorism Center, was taking over as acting secretary of defence.
The Senate would be highly unlikely to confirm a new nominee before Trump leaves office in January.
"Mark Esper has been terminated," Trump said in a tweet, adding that Miller would be acting secretary "effective immediately".
A US defence official, speaking on condition of anonymity, said White House Chief of Staff Mark Meadows called Esper to alert him Trump would be firing him through Twitter.
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.
Trump has steadfastly refused to acknowledge his election loss to President-elect Joe Biden.
Representative Adam Smith, the Democrat who leads the House Armed Services Committee, condemned Trump's decision as "childish" and reckless.
"Dismissing politically appointed national security leaders during a transition is a destabilising move that will only embolden our adversaries and put our country at greater risk," Smith said.
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.
Trump has had an uneasy relationship with the Pentagon, where Esper and top brass have repeatedly sought to avoid being seen as political instruments.
Esper's predecessor, Jim Mattis, quit in 2018 over policy differences with Trump, including on Syria.
Mattis in June criticised Trump as the "first president in my lifetime who does not try to unite the American people - does not even pretend to try. Instead he tries to divide us."
Like Mattis, Esper also disagreed with Trump's dismissive attitude toward the NATO alliance and was wary of his inclination to see US alliances through an explicitly transactional lens, sources said.