North Korean leader Kim Jong-il has died at the age of 69, state-run television has announced.
Mr Kim, who has led the communist nation since the death of his father in 1994, died on a train while visiting an area outside the capital, the announcement said.
His designated successor is his third son, Kim Jong-un, who is thought to be in his late 20s.
North Korea's state-run news agency, KCNA, urged people to unite behind the younger Kim.
A newsreader made the announcement on state-run television
"All party members, military men and the public should faithfully follow the leadership of comrade Kim Jong-un and protect and further strengthen the unified front of the party, military and the public," the news agency said.
Country representatives all over the world have been put on alert after the announcement.
South Korea's military and its National Security Council is convening for an emergency meeting, Yonhap news agency reports. The Japanese government has also convened a special security meeting.
The White House said it was "closely monitoring" reports of the death. The US remained "committed to stability on the Korean peninsula, and to the freedom and security of our allies", it said in a statement.
South Korea's President Lee Myung-Bak spoke to US President Barack Obama by telephone.
"The two leaders agreed to closely co-operate and monitor the situation together," a South Korean presidential spokesman said.
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