PYONGYANG, North Korea (AP) ― North Korea on Monday urged its 24 million people to rally behind 20-something heir-apparent Kim Jong-un as the nation mourned the death of supreme leader Kim Jong-il.

South Korea, meanwhile, put its military on high alert, while people in the streets of Pyongyang broke into tears as they learned the news that Kim had died at the age 69 of heart failure. The United States said it was in close contact with allies South Korea and Japan.

The North’s official Korean Central News Agency said the country, people and military “must faithfully revere respectable comrade Kim Jong-un.”

“At the leadership of comrade Kim Jong-un, we have to change sadness to strength and courage and overcome today’s difficulties,’’ it 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.

The North said it would place Kim’s body in the Kumsusan memorial palace in Pyongyang and would hold a national mourning period until Dec. 29. Kim’s funeral will be held on Dec. 28, it said.

In a “special broadcast’’ Monday from the North Korean capital, state media said Kim died on a train due to a “great mental and physical strain’’ on Dec. 17 during a “high intensity field inspection.” It said an autopsy was done on Dec. 18 and “fully confirmed’’ the diagnosis.

Kim is believed to have suffered a stroke in 2008, but he had appeared relatively vigorous in photos and video from recent trips to China and Russia and in numerous trips around the country carefully documented by state media.

“It is the biggest loss for the party ... and it is our people and nation’s biggest sadness,” an anchorwoman clad in black Korean traditional dress said in a voice choked with tears.

A group of North Koreans waiting at Beijing airport for their flight to Pyongyang reacted with shock when asked of the Kim’s death. One woman broke into tears as she pulled out a handkerchief. One of her companions led her away when she looked as if she would collapse.

Another man muttered, “I cant believe it. I can’t believe it. This can’t be true. We were at the embassy this morning and surely they would have known about this news.’’

He composed himself and said I wont believe it until I hear it from my own state media.

Traffic in the North Korean capital was moving as usual Monday. A foreigner contacted at Pyongyang’s Koryo Hotel said hotel staff were in tears.