Baku, December 21, AZERTAC
US Defense Secretary Jim Mattis has submitted a letter of resignation from the Trump administration, according to BBC.
Mr 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It came one day after the president's controversial announcement that US troops would be withdrawn from Syria.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, General Mattis strongly hinted his departure was caused by policy differences he had with Mr Trump.
A number of senior members of the president's own Republican party have greeted the news of Gen Mattis' resignation with concern.
Senate Majority Leader Mitch McConnell said he was "distressed" the departure was reportedly due to "sharp differences" that Gen Mattis had with the president.
Mr Trump has not immediately named a successor for the role, but said one would be appointed shortly.
As he resigned, Gen Mattis alluded to disagreements with the president in a number of policy areas.
In the letter, addressed to Mr 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, Gen 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," Gen 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".
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