South Korea launched a high alert for its military as it faces the North’s 1.2 million-strong armed forces, while President Barack Obama and South Korean President Lee Myung-bak agreed by phone to closely monitor the events and cooperate.

People on the streets of the North Korean capital, Pyongyang, broke into tears as they learned the news that their “Dear Leader” had died at the age 69 of heart failure Saturday while carrying out official duties on a train trip.

“He passed away too suddenly to our profound regret,” said a statement carried by the North’s official Korean Central News Agency. “The heart of Kim Jong Il stopped beating, but his noble and august name and benevolent image will always be remembered by our army and people.”

While there was no immediate statement on official succession, there were clear indications that Kim’s 20-something son, Kim Jong Un, would be in charge.

The North said in a dispatch that the people and the military “have pledged to uphold the leadership of comrade Kim Jong Un” and called him a “great successor” of the country’s revolutionary philosophy of juche, or self-reliance.

“At the leadership of comrade Kim Jong Un, we have to change sadness to strength and courage and overcome today’s difficulties,” North Korea said.

Kim Jong Il unveiled his third son Kim Jong Un as his successor a year ago, putting him in high-ranking posts. Little is known about the younger Kim, who is believed to be in his 20s.

Kim Jong Un regularly accompanied his father on trips around the country over the past year. Kim Jong Il inherited power after his father, revered North Korean founder Kim Il Sung, died in 1994.

Seoul and Washington will worry that Kim Jong Un “may feel it necessary in the future to precipitate a crisis to prove his mettle to other senior leaders,” according to Bruce Klingner, an Asia analyst at The Heritage Foundation think tank in Washington.

Another analyst said an internal power struggle could break out between Kim Jong Un and his aunt’s husband, Jang Song Thaek, who was elevated in the government last year and likely will be given a caretaker role in the new administration

“Tension will arise between Jang and Kim Jong Un, because Kim will have no choice but to share some power with Jang,” said Ryoo Kihl-jae, a professor at the University of North Korean Studies, which is in South Korea.