Pentagon's No. 2 official stepping down in December

Ashton Carter, the Pentagon's second-in-command, said Thursday he will step down in December, having served for two years under two secretaries of defense.

In a written statement, Carter, 59, said he had long ago set December as his intended departure but delayed his announcement because of the government's budget impasse.

"I have decided that this situation might well continue and I don't want any more time to pass" before giving Defense Secretary Chuck Hagel a chance to search for a successor," he wrote.

Hagel's press secretary, George Little, dismissed any speculation that Carter was forced out. He said Carter's decision to leave was his "and his alone."

"He'll be missed by the secretary," Little said. "They've had a strong and effective working relationship and friendship that will continue for the next two months and beyond."

Carter, a physicist and historian by training, began his tenure as deputy defense secretary in October 2011 while Leon Panetta was the Pentagon chief. For two years prior, he served as the undersecretary of defense for acquisition and technology. He also served in the Pentagon in the 1990s.

Hagel issued a statement saying he reluctantly accepted Carter's decision Thursday.

Hagel said Carter has an "unparalleled knowledge of every facet" of the U.S. defense system and will be missed. In his letter, Carter said he was ready for "my next challenge" but gave no indication what he intends to do after leaving.