President Obama is consulting with allies following the death of North Korean dictator Kim Jong Il, whose reported weekend heart attack has catapulted son Kim Jong Un to power and could further destabilize the world's most isolated nation.
"This brings extraordinary change and uncertainty to a country that has seen little change in decades. South Korea's concern is warranted, frankly, because an insecure North Korea could well be an even more dangerous North Korea," a U.S. official told Fox News.
"The most likely scenario for regime collapse has been the sudden death of Kim. We are now in that scenario," said Victor Cha, a former U.S. National Security Council director for Asian affairs.
South Korea's military and police were placed on a high alert after Kim's death and Lee convened an emergency national security council meeting.
Nearly 30,000 U.S. troops are in South Korea as part of a decades-long U.S. security agreement with that country. A Defense Departments spokeswoman said the U.S. does not discuss "specific security posture, but commanders continually assess the current force protection status and make adjustments as needed."
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Obama spoke with South Korean President Lee Myung-bak overnight Monday, according to the White House, to reaffirm "the United States' strong commitment to the stability of the Korean Peninsula and the security of our close ally."
National security officials will stay in close coordination as the situation develops, the White House said. In a separate statement, Press Secretary Jay Carney said the U.S. is also staying in touch with Japan.
According to a "special broadcast" by state media in Pyongyang late Sunday, Kim died Saturday of a heart ailment while on a train due to "great mental and physical strain" during a "high intensity field inspection." North Korea will hold a national mourning period until Dec. 29. Kim's funeral will be held on Dec. 28, the report said.
But in a demonstration of son Kim Jong Un's grasp on the communist nation, North Korea could fire off a series of missiles. The country reportedly has already launched two short-range missiles, though sources say it was a routine exercise.
While rumors of Kim's health have been circulating for several years, the leader began grooming Jong Un, his third son, a year ago to take over for him. The son, who is approximately 27, was named a general despite having never served in the military -- the goal being to exert his authority over the nation's armed services, the only fully functioning institution in the country, and the one responsible for the country's nuclear arms development program.
But Sarah McDowall, a senior analyst and desk head on Asia Pacific for IHS Jane's, a defense and security intelligence group, said Kim's decision to put Jong Un in power "could initiate a power struggle" within the military, where the average age of the generals is 80.
"There are real concerns that heir-apparent Kim Jong Un has not had sufficient time to form the necessary alliances in the country to consolidate his future as leader of the country.
Obama had been scheduled to visit South Korea in March for a nuclear non-proliferation summit, where he was to be joined by leaders from several nations. Even before that, however, the administration had been expected to decide, possibly as early as Monday, whether to try to re-engage North Korea in nuclear negotiations, providing food aid as an incentive to the starving nation. Officials who spoke to The Associated Press on condition of anonymity said Kim's death would likely delay the effort.
With Kim's death, the U.S. is also in the position of seeking to reassure China that a security commitment with South Korea is not a threat to the North's lone ally. China had been expected to host talks on Thursday in Beijing between U.S. and North Korean diplomats about restarting nuclear disarmament efforts. That meeting in turn could lead to the resumption of nuclear disarmament talks that would also include China, Japan, Russia and South Korea.
The food aid had been seen as an incentive to get North Korea to the table, In exchange, the North would agree to suspend its controversial uranium enrichment program, which would lead to six-party talks with China, Japan, Russia, the U.S. and North and South Korea.
The so-called six-party talks were last held three years ago, and resuming them would amount to a foreign policy coup for the Obama administration.
Two senior U.S. diplomats were in North Korea's lone ally China last week to discuss the issues.
They were due to meet Obama's top national security aides on Monday to discuss the way forward. Those meetings will go ahead in the wake of Kim's passing but decisions will almost certainly be delayed as it is not clear if North Korean officials will be in position to handle any engagement with the outside, the U.S. officials said.
"The possibility this week of another U.S.-DPRK bilateral meeting on the nuclear issue are probably all 'OBE' -- overtaken by events," said Cha, who is now a senior adviser and Korea chair at the Center for Strategic and International Studies. "These bits of diplomacy constituted small bites at the apple. We are now talking about the whole apple."
The Associated Press contributed to this report.