Kevin Sweeney has resigned as Pentagon chief of staff after serving the defense secretary for two years.

"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," Sweeney said in a short statement posted on the Department of Defense website late Saturday night.

A knowledgeable source told CNN's Jake Tapper that the White House forced Sweeney out. The source did not provide further information about the reason.

On Sunday, acting Defense Secretary Patrick Shanahan praised Sweeney in a pair of tweets, writing that "we're going to miss him here at the Pentagon."

"I greatly appreciate Kevin Sweeney's service to our nation, first as a Navy officer of more than 30 years, and then as the Chief of Staff for the Secretary of Defense for the past two years,"

Shanahan wrote in a tweet.

"As he steps down to reenter the private sector, we're going to miss him here in the Pentagon," he said in a second tweet. "We know great things are in store for him and his family."

Sweeney, who retired as a rear admiral in 2014, served under former Defense Secretary James

Mattis, who announced his resignation December 20 on the heels of President Donald Trump's plans to withdraw troops from Syria.

"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," Mattis wrote in his letter to the President. (CNN)