SEOUL, South Korea - The widening scope of North Korea's defiance - a rocket launch, a nuclear blast, and signs of more long-range and atomic tests to come - suggests there is more to it than the usual brinkmanship.

Ruler Kim Jong Il, looking frail and gaunt about 10 months after a reported stroke, appears anxious to secure the stability of the communist regime before he anoints a politically inexperienced son as successor.

If North Korea can win credibility as a nuclear-armed power and extract promises of recognition and aid from Washington and its allies, Kim will have set the stage for his son to take the world's first communist dynasty into its third generation.

Reading the intentions of the isolated, secretive dictatorship is never easy, but the view among analysts is that a plan is in place to name the youngest of Kim's three sons, 26-year-old Jong Un, as the future leader.

Kim, the "Dear Leader" in North Korean parlance, may have wanted to delay naming a successor until 2012, the centenary of the birth of his father, Kim Il Sung, the "Great Leader" who founded the communist state. But it is clear from an appearance in April that the ailing 67-year-old might not last that long.

Little is known about the son outside North Korea, and he has stayed out of the limelight at home, too, making it unclear how much popular support he would have. A 150-day campaign is under way to make North Koreans work harder and be prouder of their country, and it might be designed to culminate in the son's official anointment as successor in the fall.

Kim has been emphasizing science, technology, and the economy lately, and Jong Un already is being dubbed "Brilliant Comrade," one Seoul newspaper says - a sign that he will share the credit for whatever North Korea gains from its rockets and atomic bombs.

But millions of North Koreans depend on food handouts, and the pressure of sanctions is mounting. On Friday, the U.N. Security Council unanimously voted to expand an arms embargo and authorize ship searches. North Korea has little to fall back on other than its nuclear and missile programs and the possible sale of the technology to would-be nuclear nations.

North Korea detonated its first nuclear bomb in 2006 and its second last month, following it with another missile test. The regime claimed the rocket lifted a communications satellite into orbit, beating South Korea, which is to send its own rocket into space next month.

Since 1994, North Korea has played a cat-and-mouse game with the outside world, agreeing to give up its nuclear program for much-needed aid, and then reneging by claiming the other side had broken its promises.

Now, it seems to have concluded that the threat of going nuclear has not yielded concessions and that it needs to become a nuclear power.

North Korea might have enough weaponized plutonium for a half-dozen bombs, but experts say it is a few years away from mounting a nuclear warhead on a long-range missile.

"North Korea needs at least two more tests to perfect its nuclear-weapons system," said Baek Seung-joo of the Korea Institute for Defense Analyses. "It appears the North has concluded that possessing nuclear weapons is the way for it to survive."

And North Korea may be preparing a third nuclear test - possibly timed as a reaction to new U.N. Security Council sanctions, a government official in Washington said Thursday.

Caught in the middle of the standoff are two American journalists arrested at the North Korean border three months ago and sentenced to 12 years' labor for illegal entry and "hostile acts."

If North Korea ever sits down for talks with Washington to discuss a "grand plan" that includes normalizing relations, it wants to be on equal footing as a fellow nuclear power, something Defense Secretary Robert M. Gates has said Washington would never accept.

To hasten those talks and mesh them into its succession plan, North Korea is pulling out all the stops - long-range missiles, military threats, maybe another nuclear test, and holding two Americans prisoner - in a bid to strengthen its hand.