US President Donald Trump has forced Defence Secretary Jim Mattis to leave his post early, appointing an acting successor to take over in the new year.

Mattis, 68, strongly hinted at policy differences with Trump when he resigned last Friday. He offered to stay in the job until February but will now leave on January 1 after Trump reportedly balked at media coverage of his exit.

Deputy Defence Secretary Patrick Shanahan, 56, will take over the role, the US President announced on Sunday. Trump has lauded his achievements and described him as “very talented”, the BBC reported.

Shanahan, a former executive at the aerospace giant Boeing, joined the Pentagon in July 2017 after Trump nominated him.

He was reportedly a vocal supporter of the president’s plan to establish a sixth branch of the armed forces, known as the “space force”.

Originally from Washington state, Shanahan studied mechanical engineering and business at the Massachusetts Institute of Technology and joined Boeing as an engineer in 1986.

Trump had initially framed Mattis’ departure as a “retirement” but his resignation letter — full of implied criticism of the president’s foreign policy – showed that was not the case.

On Sunday morning, the hammer came down in the from of a presidential tweet.

Mattis’ replacement, Deputy Secretary Patrick Shanahan, spent most of his career working for Boeing. Having a former executive of a major defence contractor running the Pentagon, even on a temporary basis, is unusual, to say the least. It’s a job usually held by politicians with military oversight experience.

Meanwhile, the President appears to be trying to douse the fire that started this personnel crisis. He also tweeted on Sunday that the US withdrawal from Syria — abruptly announced last week — would be “slow and highly coordinated”.

Mattis resigned shortly after Trump announced his decision to withdraw all US troops from Syria.

While not mentioning it in his resignation letter directly, the general previously warned that it would be a “strategic blunder”.

In his letter, he also said the president had the right to appoint someone “whose views are better aligned with yours”. But shortly after announcing Shanahan’s appointment on Sunday, Trump moved to calm widespread concerns over the pullout which he initially said would be “rapid”.

Trump said on Twitter that he had spoken with President Recep Tayyip Erdogan of Turkey about “our mutual involvement in Syria, & the slow and highly coordinated withdrawal of US troops from the area”.

Erdogan’s office said the two leaders had agreed to “ensure coordination between their countries’ military, diplomatic and other officials to avoid a power vacuum which could result following any abuse of the withdrawal and transition phase in Syria”.