Patrick Shanahan named as acting US defence secretary

President Donald Trump has announced he will replace Defence Secretary James Mattis at the start of the new year, two months ahead of schedule.

"I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019. Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!" Trump tweeted.

Mattis announced his resignation last Thursday with a letter saying his views were not aligned with Trump, who said Mattis would retire at the end of February.

However, the veteran's last day is now expected to be January 1.

He is not expected to have a big, public farewell ceremony to try to prevent further exacerbating tensions between himself and Trump, the source said.

Trump was apparently unhappy at the extensive coverage given to Mattis' resignation and is understood to be acting promptly as a result.

Pentagon spokesman Col. Rob Manning said in a statement that "the secretary of defense serves at the pleasure of the President. The department remains focused on national security."

Mattis' retirement announcement led much of Washington, including key Republicans, to express alarm about the change in leadership at the Pentagon.

The announcement itself followed Trump's decision to call for a full withdrawal of troops from Syria and consideration of a drawdown in Afghanistan.

Shanahan, Trump's choice to replace Mattis on an interim basis, was confirmed as Mattis' No.2 in the Pentagon last year by an overwhelming margin that came after a confirmation process that included a clash with the late Arizona Senator John McCain.

Shanahan's portfolio at the Department of Defense has been narrow; he has almost exclusively handled budget issues, internal reform and the Space Force.

His official biography from the Defence Department notes Shanahan joined Boeing in 1986 and stayed with the major defence contractor for more than three decades.

He received an undergraduate degree in mechanical engineering from the University of Washington and two higher degrees from the Massachusetts Institute of Technology, according to his biography.