WASHINGTON (Reuters) -- U.S. Defense Secretary Ash Carter removed his senior military assistant, Lt. Gen. Ron Lewis, from his position on Thursday over allegations of misconduct that Carter said he learned of this week.

Carter has referred the matter to the inspector general of the Defense Department, he said in a brief statement on Thursday. After the inspector general's investigation is complete, the Army may take action, a senior defense official said.

As senior military assistant, Lewis served as a top adviser to Carter on military matters. He accompanied Carter last week on his trip to Asia.

Carter referred to "allegations of misconduct" but he did not specify the allegations. A senior Defense official also declined to specify the nature of the allegations.

"I expect the highest possible standards of conduct from the men and women in this department, particularly from those serving in the most senior positions," Carter said in his statement. "There is no exception."

Carter first learned of the allegations on Tuesday evening, and spoke to Lewis on Thursday morning, the senior defense official said.

(Reporting by Yeganeh Torbati; Writing by Mohammad Zargham; Editing by Eric Beech and Bill Rigby)