Kim Jong-Il's death has radically revamped calculations for the United States, which long waited patiently for change in nuclear-armed North Korea but is now nervous about potential dangers to come.

After years of on-off efforts to end North Korea's nuclear program, the United States recently made a tactical shift to maintain low-level dialogue as a way to discourage future provocations even if no big issues are resolved.

The United States had been expected this week to decide whether to step up engagement through new talks and possible food aid. But the situation changed instantly with North Korea now led not by a recalcitrant strongman but an untested young leader.

Kim Jong-Il, 69, had been groomed for 14 years as successor to his father, the regime's founder Kim Il-Sung. Heir apparent Kim Jong-Un is in his late 20s and is believed to lack a firm support base within the opaque regime.

"Kim Jong-Il picked the apple that didn't fall far from the tree. He didn't select a successor who he believed would radically depart from his vision for North Korea," a US official said on condition of anonymity.

The official expected the regime to "try to convey a sense of order" during the mourning period and transition. But the longer-term calculations are more difficult to predict.

"A lot depends on whether the power centers of the regime coalesce around Kim Jong-Un, or see this period of uncertainty as an opportunity to change the balance of power internally," the official said.

North Korea expert L. Gordon Flake, executive director of the Mansfield Foundation, said that US diplomacy is dependent on policy, not personalities, but that Pyongyang is unlikely to be able to make key decisions right now.

But Flake, who advised then-senator Barack Obama during his presidential campaign, said there was no reason to want the status quo. Kim Jong-Il set off repeated crises since inheriting power in 1994, including carrying out two tests of nuclear weapons.

"Some people thought for his entire reign that we're just waiting around the corner for Kim Jong-Il to be some type of reformer. That obviously didn't pan out," he said.

"In the short run, there is the risk that North Korea may lash out. But in the long run, I don't think there's any way to bemoan Kim Jong-Il's passing," he said.

Secretary of State Hillary Clinton had been due to hold meetings Monday on whether to approve food assistance to impoverished North Korea, which has long sought help for what aid groups call a serious humanitarian situation.

But Clinton instead huddled with aides over Kim's death. She urged a "peaceful and stable" transition and hoped for better relations with "the people of North Korea."

Despite its policy worldwide of not closing the door on talks with US adversaries, the Obama administration has been adamant that it will not resume formal negotiations until North Korea clearly commits to past agreements on denuclearization.

While Kim's death sent shockwaves throughout the world, the news was not unexpected. He suffered a stroke three years ago and, with his reputed passion for fatty foods and alcohol, was certainly not known for a healthy lifestyle.

In a presentation last year, a military strategist warned that the United States needed to study all possible outcomes as a complete collapse of the nuclear-armed regime could trigger a crisis unseen since World War II.

Colonel David Maxwell of the Army's Special Operations Command, speaking in personal capacity, said in the presentation that North Koreans should be expected to resist fiercely any foreign forces and could mount an insurgency far more sophisticated than those seen in Iraq or Afghanistan.

Former New Mexico governor Bill Richardson, who has maintained contacts with North Korea since his time as US ambassador to the United Nations, said that fellow Democrat Obama is "playing it correctly by playing it cool."

"You got to watch things in the next 48 hours. I think what the North Korean military commanders say in the next day or so is going to be critical," Richardson told CBS television.

Some members of the rival Republican Party urged regime change. Senator John McCain voiced satisfaction that Kim was "in a warm corner of hell" and, while urging caution, called for "determined and creative leadership" to end his regime.

Representative Ed Royce, a Republican who has led a push against US food assistance, said the transition "is less stable than may appear" as "the North Korean people are starting to question this corrupt dynasty."

"Now is not the time for talk of new beginnings and food aid. We should be doing what we can to delegitimize this succession with the suffering North Korean people," Royce said.