File photo shows the U.S. Defense Secretary James Mattis at the Pentagon, Virginia, the United States, on Aug. 7, 2018. U.S. President Donald Trump said Thursday that Mattis is to retire at the end of February.Mattis will "be retiring, with distinction," after having served the administration as the defense chief for the past two years, Trump tweeted. "During Jim's tenure, tremendous progress has been made, especially with respect to the purchase of new fighting equipment," the president said. He added that a new secretary of defense "will be named shortly." (Xinhua/Liu Jie)

U.S. President Donald Trump said Thursday that Defense Secretary Jim Mattis is to retire at the end of February.

Mattis will "be retiring, with distinction," after having served the administration as the defense chief for the past two years, Trump tweeted.

"During Jim's tenure, tremendous progress has been made, especially with respect to the purchase of new fighting equipment," the president said.

He added that a new secretary of defense "will be named shortly."

There have been reports that the relationship between Trump and Mattis has frayed, but the president didn't mention his disputes with Mattis or whether he was forced to resign.

However, in a resignation letter to Trump dated Thursday, the defense secretary wrote "because you have a right to have a Secretary of Defense whose views are better aligned with yours" on many subjects, "I believe it is right for me to step down from my position."

In the letter, the 68-year-old former Marine Corps general implicitly criticized Trump for his treatment of U.S. allies.

"One core belief I have always held is that our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships," he wrote.

He added that his views on "treating the allies with respect" are "strongly held and informed by over four decades of immersion in these issues."

The announcement of Mattis's departure came one day after he ordered the withdrawal of the 2,000 or so U.S. troops from Syria and declared victory over the Islamic State (IS) militant group.

Mattis has advocated a longer-term U.S. military presence in Syria.

Republican Senator Lindsey Graham, who opposed Trump's decision to bring back U.S. troops in Syria, said Mattis "firmly" believes "the job in Syria is not yet done," according to a report by Fox 31, a channel affiliated to the Fox TV.