Washington: US Defence Secretary Jim Mattis has abruptly said he is quitting after falling out with President Donald Trump over his foreign policies, one day after Trump rebuffed top advisers and decided to pull all US troops out of Syria.

Mattis announced plans to resign after a face-to-face meeting with Trump on Thursday in which they aired their differences, a senior White House official says.

"Because you have a right to a Secretary of Defence 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 said in his resignation letter, released by the Pentagon.

Trump announced on Wednesday that US troops in Syria would be withdrawn, a decision that upended American policy in the region.