US Defence Secretary James Mattis is resigning, the latest in a string of senior US officials to do so, the BBC reports. 

His announcement came a day after President Trump said he was withdrawing troops from Syria - a decision General Mattis is understood to oppose.

In his resignation letter, Gen Mattis strongly hinted at policy differences with Mr Trump.

General Jim Mattis will be retiring, with distinction, at the end of February, after having served my Administration as Secretary of Defense for the past two years. During Jim’s tenure, tremendous progress has been made, especially with respect to the purchase of new fighting....

— Donald J. Trump (@realDonaldTrump) December 20, 2018

He said the president had the right to appoint someone "whose views are better aligned with yours".

Gen Mattis, 68, will leave the job in February. President Trump has not immediately named a successor, but said one would be appointed shortly.