SEOUL, South Korea -- South Korea could see more North Korean provocations as Pyongyang works to strengthen leader Kim Jong Il's successor, the top U.S. military officer said Thursday.
Seoul says North Korean attacks last year killed 50 South Koreans as Kim Jong Il's third son Kim Jong Un rose to prominence as heir-apparent.
"A succession plan being executed has been ongoing for some time, and that's not an insignificant part of the whole provocation cycle," Adm. Mike Mullen told reporters in Seoul.
The chairman of the U.S. Joint Chiefs of Staff also said that North Korea's "threat remains very real" as it continues to pursue improved nuclear capabilities.
"I'm not convinced that they won't provoke again. I've said for a long time that the only thing predictable about North Korea is their unpredictability," Mullen said.
Mullen's visit to South Korea comes amid high tension between the Koreas. Seoul blames a North Korean torpedo for the sinking of a South Korean warship in March 2010 that killed 46 sailors; North Korea denies any role. North Korea shelled a front-line South Korean island in November, killing four people.
North Korea also threatened last month to retaliate for the South Korean military's use of photos of Kim Jong Il and his family for shooting practice.
"We have a sense of urgency to essentially work on planning to deter the North from further provocations. Whether they will be deterred or not, that's to be seen," Mullen said.
Mullen came to Seoul after a trip to China, the North's major ally, where he expressed worry over Beijing's increasing military capabilities and discussed North Korea with Chinese leaders.
"The emphasis was on stability. It was, very specifically, stability in North Korea and the responsibility we all have in this case, the United States and China, to do all we can," Mullen said.
Mullen urged China to play a "leadership role" in restraining Pyongyang.
"China certainly has influence in Pyongyang, but ... it's not an infinite amount of influence. I think we have to understand that challenge," he said.
Earlier Thursday, U.S. Army Gen. James Thurman became the new commander of the 28,500 U.S. troops in Korea, replacing Gen. Walter Sharp.
Thurman said the countries' "alliance stands ready to counter any provocation intended to destabilize the Korean peninsula."
South Korea handed over control of its troops to the United States in 1950 after the start of the Korean War. It regained peacetime control in 1994, but the top U.S. general in Korea is still supposed to command Korean forces if war breaks out. That's scheduled to change in 2015.
U.S. forces in South Korea are a legacy of the Korean War, which ended with a truce rather than a peace treaty. The two Koreas are separated by a heavily militarized frontier.