WASHINGTON — The top U.S. military officer described the situation in Afghanistan as "serious and deteriorating," but refused to say Sunday whether defeating a resilient enemy would require more than the 68,000 American troops already committed. Adm. Mike Mullen also expressed concern about eroding public support as the U.S. and NATO enter their ninth year of combat and reconstruction operations. The comments from the chairman of the Joint Chiefs of Staff underscore the challenges that the U.S.