North Korea's neighbours and its old enemy, the United States, appealed for stability on Monday after the death of the totalitarian state's leader, Kim Jong-il, plunged one of the world's most heavily militarised regions into fresh uncertainty.

Kim suffered a heart attack while travelling on a train, North Korean state media said, setting up the autocratic, well-armed and nuclear-ambitious state for only its second leadership change since the Korean war ended in an uneasy truce in 1953.

Later, state media anointed his youngest son, Kim Jung-un, as the great successor, but little is known about how he would run a reclusive state with more than a million troops and missiles that Washington fears could one day reach U.S. shores.

U.S. President Barack Obama and South Korean and Japanese leaders spoke by telephone to discuss the situation, which was unfolding well into Washington's night, while the North's biggest ally, China, expressed grief over Kim's sudden death.

South Korea, still technically at war with the North, put its military on alert after the news, Yonhap news agency said, and President Lee Myung-bak convened its National Security Council. Seoul's Defence Ministry said there were no signs of any unusual North Korean troop movements.

The White House released a short statement committing itself to working with both South Korea and Japan, two of its closest Asian allies, to ensure continued stability in the region.

We remain committed to stability on the Korean peninsula, and to the freedom and security of our allies, White House press secretary Jay Carney said in a short written statement.

Japan also convened a special security meeting but stopped short of putting its own armed forces on special alert, and said it had to be prepared for unexpected security developments.

Prime Minister (Yoshihiko) Noda instructed ministers at the security meeting to prepare for the unexpected, including financial matters, North Korea's domestic issues and border affairs, Japan's top government spokesman said on Monday.

Noda told reporters it was important that Kim's death did not adversely affect peace on the Korean peninsula. He also said there was no change in Japan's demand for the return of Japanese citizens abducted by North Korean agents decades ago.

JAPAN NERVOUS OVER MISSILES

Japan is acutely sensitive to uncertainty on the Korean peninsula, after Pyongyang has repeatedly tested missiles into the sea between them and also at times over Japan itself.

There was initially a yawning four-hour silence from North Korea's main ally, China, before it praised Kim Jong-il as a great leader who made important contributions to relations with Beijing, a friendship that had helped to maintain stability.

China and North Korea will strive together to continue making positive contributions to consolidating and developing the traditional friendship between our two parties, governments and peoples, and to preserving the peace and stability of the Korean peninsula and the region, state news agency Xinhua quoted Foreign Ministry spokesman Ma Zhaoxu as saying.

Without China's support, many experts believe the Kim dynasty would collapse. Russia, another neighbour with a stake in a stable Korea, had yet to comment on the news.

In Europe, British Foreign Secretary William Hague reflected the West's hope that a change in leadership could draw the reclusive state out of isolation.

This could be a turning point for North Korea. We hope that their new leadership will recognise that engagement with the international community offers the best prospect of improving the lives of ordinary North Korean people, he said.

EU foreign policy chief said the European Union was monitoring the situation.

China, Russia, the United States, Japan, South Korea and North Korea make up the so-called six-party talks, a troubled dialogue aimed at persuading Pyongyang to scrap nuclear programmes and give up ambitions to develop nuclear weapons.

The talks collapsed in 2008, though there have been some fruitless diplomatic efforts since then to revive them.

Australia, which fought for the South in the Korean war and maintains diplomatic relations with Pyongyang, called for calm, especially around the demilitarised zone separating the Koreas.

It's at times like this that we cannot afford to have any wrong or ambiguous signalling, said Foreign Minister Kevin Rudd, who visited the heavily fortified border zone last month.

This is the single largest militarily armed zone anywhere in the world and we need to ensure that calm and restraint are exercised in an exceptionally difficult period of transition.

Taiwan set up an emergency committee within its Foreign Ministry to monitor developments on the Korean peninsula.

(Reporting by Reuters bureaux; Writing by Mark Bendeich; Editing by Ed Davies, Robert Birsel and Peter Graff)