By Jennifer Rizzo and Shirley Henry

Joint Chiefs Chairman Gen. Martin Dempsey said Wednesday he clearly understands a complete withdrawal of U.S. troops from Afghanistan by the end of 2014 - known as the "zero option" - is a "possibility" given Afghan President Hamid Karzai's current unwillingness to sign the security pact that would govern some troops remaining in his country beyond that time frame.

But Dempsey also said he has not been told to plan for such a withdrawal.

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