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 Mr Trump rebuffed top advisers and decided to pull all US troops out of Syria.

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

“Because you have a right to 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,” Mr Mattis said in his resignation letter, released by the Pentagon.

Here's Defense Secretary Jim Mattis' letter of resignation. It comes one day after President Trump announced he would pull U.S. troops from Syria pic.twitter.com/5yyKr6McYk

— Marcus Weisgerber (@MarcusReports) December 20, 2018

Mr Trump announced on Thursday (Wednesday local time) that US troops in Syria would be withdrawn, a decision that upended American policy in the region.

On Thursday, officials said the president was considering a substantial US pullout from the 17-year-long conflict in Afghanistan.

Mr Mattis had opposed the decision on Syria, an official said.

His letter indicated he disagreed with Mr Trump’s isolationist policies, writing it was his belief the United States needed to maintain strong alliances and show allies respect.

Mr Trump has withdrawn the United States from several international agreements since taking office in January 2017.

Mr Trump said on Twitter he would nominate a successor to Mr Mattis shortly.

“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,” Mr Trump posted on Thursday.

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

“General Mattis was a great help to me in getting allies and other countries to pay their share of military obligations. A new Secretary of Defense will be named shortly. I greatly thank Jim for his service!”

The two men met in the Oval Office on Friday (Thursday afternoon local time).

During the session, Mr Mattis told Mr Trump of his plans to resign, the senior White House official said.


”He and the president had differences on some issues. I don’t know if it was specifically Syria,” the official said.

Mr Mattis joins a long list of former Trump administration senior figures who have either quit or been removed, some unceremoniously such as secretary of state Rex Tillerson, who Mr Trump fired via Twitter in March.

Mr Trump’s White House has had the highest turnover of senior-level staff of the past five presidents, according to the Brookings Institution think tank.

Speculation that Mr Mattis might not last long in his post grew in October when Mr Trump said in a CBS interview the general was “sort of a Democrat” and might be leaving.

Mr Mattis, along with other national security aides, was said to have opposed Mr Trump’s decision to remove US troops from Syria.

Many US lawmakers have expressed concern about the decision and asked Mr Trump to reconsider.

Mr Mattis has argued for maintaining a strong US military presence in Afghanistan to bolster diplomatic peace efforts.

The Pentagon declined comment on Afghanistan, while a National Security Council spokesman said the White House was not going to comment “on future strategic developments”.

-AAP