Adm. Mike Mullen said North Korea has raised the ante in its aggression against South Korea.
Mullen told troops in Iraq that the old tit for tat pattern with North Korea has changed.
"It's changed out there, and it's dangerous. Increasingly dangerous," Mullen said during an informal question and answer session with troops.
He said the North's provocations are tied to preparations for Kim's son to take power.
N. Korea Threatens Nuclear War over Arms Drills
Top U.S. Officer Warns N. Korea against Attacks
South Korea to Create Island "Fortresses"
S. Korea Holds Naval Drills Amid Diplomacy Rush
N. Korea Lambasts S. Korea's New Defense Chiefs
China is the only nation with the power to rein in North Korea, and it should do so, Mullen said. He made that point during visits to South Korea and Japan last week. China has so far done little to condemn its traditional ally for actions that include the sinking of a South Korean warship and the shelling of a disputed island.