General David Petraeus, the head of the U.S. command in Afghanistan, is leaving at the end of the year, the Times of London reported Tuesday. Sources said there is already a search under way for a successor to Petraeus, who took over the top position last July after Gen. Stanley McChrystal resigned. The Pentagon, denied the rumor, however, saying Petraeus would "cycle out" at a date that has not been determined. Petraeus has been rumored as a successor to Adm. Mike Mullen, chairman of the Joint Chiefs of Staff who is expected to retire in October. Defense Secretary Robert Gates is also tapped to retire this year. The shakeup among the nation’s top military leadership leaves a huge question mark for the Obama administration's Afghanistan policy; the president has promised to begin withdrawing troops as soon as this summer.