Kevin M. Sweeney Image Credit: Photo courtesy: Department of Defense

Rear Adm. Kevin M. Sweeney has resigned his post as chief of staff to the US secretary of defence, the Defense Department said on Saturday.

Sweeney had become chief of staff to former Defense Secretary Jim Mattis in January 2017, and his resignation comes soon after Mattis' own pointed departure in December. The two had known each other for years.

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Mattis said then that he had resigned in protest of President Donald Trump's decision to withdraw US forces from Syria and in protest of the president's rejection of international alliances.

Mattis had originally said he would step down at the end of February, but angered by Mattis' criticism, Trump said he was removing Mattis from his post two months early.

Patrick M. Shanahan, the former deputy secretary of defence, replaced Mattis and became acting secretary of defense on January 1.

"After two years in the Pentagon, I've decided the time is right to return to the private sector," Sweeney said in a statement on the department's website. "It has been an honor to serve again alongside the men and women of the Department of Defense."

A spokeswoman for the department said Saturday that she did not have any further comment beyond the statement. Sweeney could not immediately be reached for comment.

After two years in the Pentagon, I've decided the time is right to return to the private sector. It has been an honor to serve again alongside the men and women of the Department of Defense.

- Rear Adm. Kevin M. Sweeney

Sweeney retired as a rear admiral from the US Navy in 2014, according to the Department of Defense. The department said as commanding officer of the USS Cole, he had overseen the destroyer's restoration.

He had also served as executive assistant to the undersecretary of the Navy and executive assistant to the commander of the Joint Forces Command.

After his retirement from active duty, Sweeney spent time working in various executive roles in the private sector, according to the Defense Department. He graduated from the Naval Academy in 1982.

Sweeney's resignation follows a number of recent high-profile departures from the Trump administration, including that of Mattis. The president said in December that John F. Kelly would step down as White House chief of staff. Brett McGurk, the special presidential envoy to the coalition fighting Daesh, also announced his resignation last month.Pentagon Chief of Staff Resigns