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."

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...