Donald Trump on Sunday announced he would replace Defence Secretary Jim Mattis with his deputy Patrick Shanahan, days after the outgoing Pentagon chief quit while citing key policy differences with the US president, notably on Syria.

“I am pleased to announce that our very talented Deputy Secretary of Defence, Patrick Shanahan, will assume the title of Acting Secretary of Defence starting January 1, 2019,” the Republican leader tweeted, accelerating Mattis' planned departure by two months.

“Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!”

Mattis, 68, had said he would leave at the end of February to allow a smooth transition for the next chief of the world's top military power — but Trump sped the process amid reports that he was upset over media coverage of the stinging resignation letter submitted by Mattis.

The announcement that Mattis would leave the administration came just after Trump stunned Washington and allies abroad in declaring that US troops would leave Syria and significantly withdraw from Afghanistan.

Mattis and others had strongly counseled the often impulsive president against those moves — and the decorated retired general did little to hide his disagreement with Trump.

“Because you have the right to have a secretary of defence whose views are better aligned with yours,” Mattis said in the letter, “I believe it is right for me to step down from my position.”

“My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues,” Mattis wrote.

Shanahan spent over three decades working for aircraft giant Boeing, including as vice president and general manager of Boeing Missile Defence Systems, before moving to the Pentagon as deputy in 2017.