US Defence Secretary Jim Mattis resigns

US Defence Secretary Jim Mattis has submitted a letter of resignation from the Trump administration.

Trump tweeted the news on Thursday, saying the general will retire "with distinction" at the end of February.

It came one day after the president's controversial announcement that US troops would be withdrawn from Syria.

In his letter, Mattis strongly hinted his departure was caused by policy differences he had with Trump.

A number of senior members of the president's own Republican party have greeted the news of Mattis' resignation with concern.

Senate Majority Leader Mitch McConnell said he was "distressed" the departure was reportedly due to "sharp differences" that Mattis had with the president.

Trump has not immediately named a successor for the role, but said one would be appointed shortly.

As he resigned, Mattis alluded to disagreements with the president in a number of policy areas.

In the letter, addressed to Trump directly, he described his views on "treating allies with respect" and using "all the tools of American power to provide for the common defence".

"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," he wrote.

While not mentioning the Syria troop pull-out directly, Mattis had previously warned that an early withdrawal from the country would be a "strategic blunder".

He also appeared to point to differences on a number of other key issues, like Russia and Nato.

"My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues," Mattis wrote in his letter.

He also confirmed he would continue in the role until the end of February to "allow sufficient time for a successor to be nominated and confirmed".

Members of congress from both side of the political divide have reacted with shock to the resignation.

Florida Senator Marco Rubio, who ran against Trump for the Republican nomination in 2016, said the letter "makes it abundantly clear that we are headed towards a series of grave policy errors which will endanger our nation, damage our alliances & empower our adversaries".

McConnell, the Republican's Senate leader, issued his own statement responding to the shock announcement.

John Kasich, Ohio's Republican governor, described current events surrounding the Trump administration as "chaos".

Democratic Senator Mark Warner, who is vice chair of the Senate Intelligence Committee, labelled the development "scary".

He described Mattis as "an island of stability amidst the chaos of the Trump administration".

House Minority Leader Nancy Pelosi told US media she was "sad" and "shaken" by the news.

She also described Gen Mattis as a "comfort to many of us as a voice of stability in the Trump administration."