The chairman of the U.S. Joint Chiefs of Staff Michael Mullen says North Korea is a big concern for the United States and stressed the need to closely monitor the communist country.


In a speech at a college in Pennsylvania Monday, Mullen said North Korea is a "huge challenge and a great concern to the U.S."

But, the top U.S. military officer expressed satisfaction with the capability of the South Korean military, noting that the transfer of wartime control of Korean troops to Seoul is under way.