WASHINGTON (PTI): US President Donald Trump on Sunday announced that he would replace Defence Secretary Jim Mattis with his deputy Patrick Shanahan as acting Pentagon chief on January 1.

Mattis resigned last week over his differences with Trump on key policy issues, especially on his decision to withdraw US troops from Syria.

"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," Trump tweeted.

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

Shanahan spent over three decades working for aircraft giant Boeing before moving to the Pentagon as deputy in 2017.

Mattis' resignation came just after Trump declared that US troops would leave Syria, a decision termed by many experts and lawmakers as "rash" and "dangerous".

In his resignation letter, Mattis wrote, "Because you have the right to have a secretary of defense whose views are better aligned with yours, 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."

Trump also said he had a long and productive call with his Turkish counterpart.

"We discussed ISIS, our mutual involvement in Syria, & the slow & highly coordinated pullout of US troops from the area," he tweeted.