Governments around the world were tonight watching warily developments in North Korea following the sudden death of Kim Jong Il.
Cautious optimism that the demise of the 69-year-old dictator could usher in a new era of change was tempered by uncertainty as to how the notoriously unpredictable regime would react.
Foreign Secretary William Hague voiced the hope that the end of Kim's 17-year despotic reign could mark a "turning point" in the history of the impoverished, nuclear-armed, Stalinist state.
In contrast, South Korea - which remains technically at war with its neighbour - put its forces on a state of high alert amid fears the regime could react with a show of strength.
State media in Pyongyang signalled power would now pass to Kim's third son, Kim Jong Un, hailing him as the "great successor".
Little is known in the West about the younger Kim, who is thought to be in his late 20s and was educated in Switzerland.
Since last year he has been publicly groomed as heir apparent to his father, who suffered a stroke in 2008, and it had been thought there would have been further moves to prepare him to assume the mantle of power during 2012.
Mr Hague urged the new leadership to end the country's years of virtual isolation and engage with the rest of the world, although he admitted it was "difficult to be optimistic" that the country would change.
"This could be a turning point for North Korea," he said in a statement.
"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."
Mr Hague also called on Pyongyang "to work for peace and security in the region" and to allow the resumption of the regional six party talks on the denuclearisation of the Korean peninsula.
The White House said President Barack Obama had spoken to South Korea's President Lee Myung-bak and voiced the United States' "strong commitment" to the security of "our close ally".
"The two leaders agreed to stay in close touch as the situation develops and agreed they would direct their national security teams to continue close co-ordination," the White House said in a statement.
The South Korean news agency Yonhap reported that North Korea had carried out at least one test-firing of a short-range missile, although it quoted officials as saying it was unlikely to have been linked to Kim's death.
In Japan - which shares South Korea's anxieties about Pyongyang's intentions - Prime Minister Yoshihiko Noda summoned an emergency meeting of the national security council.
Chief cabinet secretary Osamu Fujimura said he had instructed them "to be best prepared in case of any unexpected development".
"First of all we hope that this sudden development would not give adverse impact on the peace and stability of the Korean peninsula," he said.
In China, North Korea's most important ally and diplomatic protector, Foreign Ministry spokesman Ma Zhaoxu paid tribute to Kim as a "great leader".
He said China believed the North Korean people would "turn their grief into strength, unite as one, and continue to advance the cause of North Korean socialism".
Beijing would, he added, continue to offer its support and make "active contributions to peace and stability on the Korean peninsula and in this region".
State television in North Korea broadcast scenes of mourning, with many people crying uncontrollably for the "dear leader" who in life had demanded absolute devotion from his people.
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