Trump described Shanahan as a 'very talented' individual.

He will take charge from Jan 1.

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

The appointment has been made just days after US Defense Secretary James Mattis resigned from the position.

According to media reports, Mattis resigned from the position amid differences with US President  Donald Trump over plans to withdraw troops from Syria.

"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," Mattis wrote in his letter to the President as reported by CNN.

A senior administration official told CNN that Mattis was "vehemently opposed" to the Syria decision and a possible Afghanistan troop withdrawal. 

He offered to stay in the job until February but will now leave on 1 January after Trump reportedly balked at media coverage of his exit, reported BBC.

Image: Donald Trump Facebook page