"Mark Esper has been terminated. I would like to thank him for his service," Trump said on Twitter.

He announced that Christopher Miller, the current head of the National Counterterrorism Center and former special forces officer, would replace Esper as acting defense secretary

Esper, Trump's fourth Pentagon chief in four years, was fired after 16 months in the job trying to keep his head down politically as he pursued fundamental reforms to the massive Pentagon bureaucracy and sought to reshape the US defense posture worldwide to focus on the threat of China.

But Trump was angered when Esper resisted pressure to deploy federal troops to quash civil unrest, and also slow-walked Trump's desire for a rapid full withdrawal of US troops from Afghanistan while violence continues in the country.

Trump's move, expected by many insiders, was nevertheless a shock, coming just a week after losing his bid for reelection to Democrat Joe Biden and just 10 weeks before he will depart the White House.

It added also adds to a sense of lowered security in the country, as Trump refuses to accept the verdict of last Tuesday's election and has pledged to reverse it in court.

Miller spent 31 years in the army, deploying in Afghanistan in 2001 and Iraq in 2003 with the special forces.

After retiring, he became a consultant on clandestine operations and intelligence to the government.

In 2018-2019 he was a White House advisor on counterterrorism and transnational threats, and from 2019 was deputy assistant secretary of defense for special operations.

In August he was named director of the National Counterterrorism Center.