Pierre Verdy, AFP | In this file photo taken on June 16, 2009 Patrick Shanahan, vice president and general manager of Airplane Programs for Boeing Commercial Airplanes, addresses a press conference during the 48th international Paris Air Show.

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

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"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," the Republican leader tweeted, accelerating Mattis's planned departure by two months.

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

Trump last week ordered a complete troop pullout from Syria, asserting that the Islamic State group had been defeated, as well as a significant withdrawal from Afghanistan.

Analysts said the momentous reversal of years of US foreign policy will leave the war-torn regions at risk of continued and potentially heightened bloodshed.

Mattis, 68, was one of Trump's first cabinet picks and has spent nearly two years at the Pentagon.

In his resignation letter Mattis made clear that Trump's world view was irreconcilable with his own.

"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' immersion in these issues," Mattis wrote to Trump, who has sought closer ties with Russia and heaped contempt on NATO and other alliances.

"Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position."

Brett McGurk, the US special envoy to the anti-IS coalition, also announced he would bring his departure forward from February due to the Syria policy change.

(AFP)

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