Donald Trump, the US president, on Sunday said he was replacing Defense Secretary Jim Mattis on Jan. 1 with deputy defense secretary Patrick Shanahan in an acting capacity after Mr Mattis resigned over policy differences and offered to stay in his post for two more months.

A source familiar with the situation told Reuters that Trump was irked by the attention given to Mr Mattis’ resignation letter, which implicitly criticized Trump’s foreign policies and his treatment of military allies after the president’s decision to pull troops out of Syria.

Mr Trump announced Mr Shanahan’s appointment in a tweet, calling the former Boeing Co executive "very talented".

In a shock announcement on Wednesday, Trump said he was withdrawing US troops from Syria, a decision that upended American policy in the region.

Mr Trump expedited Mr Mattis’ departure by tweeting on Sunday.

"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 Mr Mattis’s planned departure by two months.

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

This is a developing story, more to follow