By Phil Stewart and Steve Holland

WASHINGTON (Reuters) - U.S. Defense Secretary Jim Mattis, a stabilizing force in President Donald Trump's Cabinet, abruptly announced his resignation on Thursday and said Trump should pick a successor whose views align more with his own.

Mattis' resigned a day after Trump announced that U.S. troops in Syria would be withdrawn, a decision that upended American policy in the region, and on the same day that officials said the president was considering a substantial U.S. pullout from the long-running conflict in Afghanistan.

"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," Mattis said in his resignation letter, released by the Pentagon.

Trump, announcing Mattis' departure on Twitter, said he would nominate a successor soon.

"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," he said.

Mattis joins a long list of former Trump administration senior figures who have either quit or been removed, some unceremoniously like Secretary of State Rex Tillerson who Trump fired via Twitter in March.

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

Mattis, along with other national security aides, was said to have opposed Trump's decision to remove U.S. troops from Syria. Many U.S. lawmakers have expressed concern about the decision and asked Trump to reconsider.

(Reporting by Phil Stewart and Steve Holland in Washington; writing by Tim Ahmann; editing by James Dalgleish and Grant McCool)

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