Chairman of the US Joint Chiefs of Staff Admiral Mike Mullen said he is concerned about why China's military is focusing some of its emerging capabilities on the United States. He spoke in Washington after US Defense Secretary Robert Gates completed a trip to China calling for greater military cooperation. Admiral Mullen said he understood China's need to improve its military and he was not surprised by China's first known test flight of a stealth fighter on Tuesday. He also raised concerns about the specter of cyber warfare targeting the United States.