N Korean leader Kim Jong-il dies

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 BBC reported.

He suffered a stroke in 2008 and was absent from public view for months.

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.

State media also referred to him as the "great successor to the revolutionary cause" in what appeared to be the first such reference.

A funeral for Kim Jong-il will be held in Pyongyang on 28 December and Kim Jong-un will head the funeral committee, KCNA reports. A period of national mourning has been declared from 17 to 29 December.

China - North Korea's closest ally and biggest trading partner - said it was "distressed" to hear the news of his death.

South Korea's military has been put on alert following the announcement 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.

Asian stock markets fell after the news was announced.