(Adds McCain comments, rewrites top to focus on nuclear program)
WASHINGTON, Dec 15 (Reuters) - U.S. Secretary of State John Kerry described North Korea's Kim Jong Un as reckless and insecure after the execution of the leader's powerful uncle, and said Kim's actions underscored a need for a unified stand against Pyongyang's nuclear program.
The execution of Jan Song Thaek, considered the second most powerful man in the secretive country, showed why China, United States and other countries must work together to limit North Korea's nuclear weapons development, Kerry said in the interview on ABC's "This Week" program aired on Sunday.
North Korean state media on Friday reported the execution of Jang. North Korea said earlier it had stripped Jang of his power and positions and accused him of criminal acts including mismanagement of the state financial system, womanizing and alcohol abuse.
North Korean politics are virtually impenetrable from outside and Jang also could have been purged over a falling out with Kim or other personal reasons.
"It tells us a lot about, first of all, how ruthless and reckless he is," Kerry said of Kim. "And it also tells us a lot about how insecure he is, to a certain degree.
"The insights that we have tell us that he is spontaneous, erratic, still worried about his place in the power structure, and maneuvering to eliminate any potential kind of adversary or competitor and does so obviously ruthlessly."
The top U.S. diplomat, in some of the most detailed remarks of a U.S. official since the news on Friday, said the execution was not the first under Kim's rule and pointed to the urgency of addressing the North Korean nuclear state.
"It tells us a significant amount about the instability internally of the regime, with the numbers of executions," Kerry said. "It's an ominous sign of the instability and of the danger that does exist."
The young Kim, believed to be about 30, has carried out two long-range missile tests and a nuclear weapons test in defiance of U.N. sanctions since he took control two years ago after the death of his father, Kim Jong Il.
The Obama administration is working with China, the closest thing Pyongyang has to an ally, in seeking help to prevent any internal upheaval in North Korea from destabilizing the Korean peninsula, U.S. officials say.
Kerry, in the interview, said the nature of "this ruthless, horrendous dictatorship" and Kim's insecurities raised the stakes for China, Russia, Japan, South Korea and the United States to "stay on the same page" and push ahead on denuclearization.
"To have a nuclear weapon potentially in the hands of somebody like Kim Jong Un just becomes even more unacceptable," Kerry told ABC.
Senator John McCain, a leading Republican voice on foreign policy issues, echoed Kerry's concern about the threat posed by Kim's latest behavior and called on China to step in.
"They've got to rein this young man in, and they can," McCain said on CNN's "State of the Union" program on Sunday.
"I think it's pretty obvious this young man is capable of some very aberrational behavior and given the toys that he has, I think it's very dangerous." (Reporting by Alina Selyukh; Editing by Bill Trott)