Media playback is unsupported on your device Media caption Scenes of mourning for Kim Jong-il have been broadcast on state television

Strange natural phenomena have been witnessed in North Korea since the death of the country's leader Kim Jong-il, the state news agency KCNA reports.

Ice cracked on a famous lake "so loud, it seemed to shake the Heavens and the Earth", and a mysterious glow was seen on a revered mountain top, KCNA said.

The personality cult surrounding North Korea's founding father and son bestows near-divine status on them.

Meanwhile, South Korean intelligence has questioned how Mr Kim died.

Citing US satellite photos, the country's national intelligence service director Won Sei-hoon said there was no sign that the special train, on which Mr Kim is reported to have died while on a visit on Saturday, had ever left Pyongyang over the weekend.

South Korea's president says Seoul is trying to show no hostility towards Pyongyang. Lee Myung-bak said a return to stability in the communist state was in the interests of the region.

Seoul is also sending its top nuclear negotiator to Beijing for talks on the situation in its northern neighbour.

In North Korea, state media continue to report mass grieving following Mr Kim's death, reportedly from a heart attack.

'Message in rock'

The 69-year-old had led North Korea since the death of his father in 1994 and an elaborate personality cult, involving multiple stories of alleged miracles or astonishing deeds, has been built up around him.

Even nature is mourning, the state-run Korean Central News Agency reported on Thursday.

North Korea Image caption North Korean leader Kim Jong-il had suffered from poor health since his stroke in 2008 Population about 23 million

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A snowstorm hit as Mr Kim died and ice on the volcanic Chon lake near his reported birthplace at Mount Paektu cracked, it said.

Following the storm's sudden end at dawn on Tuesday, a message carved in rock - "Mount Paektu, holy mountain of revolution. Kim Jong-il" - glowed brightly, it said. It remained there until sunset.

On the same say, a Manchurian crane also apparently adopted a posture of grief at a statue of the late leader's father in the northern city of Hamhung.

"Even the crane seemed to mourn the demise of Kim Jong-il, born of Heaven, after flying down there at dead of cold night, unable to forget him," KCNA reported officials as saying.

On Wednesday state media said more than five million people had already turned out to pay their respects to Kim Jong-il.

State media have called on North Koreans to unite behind his designated heir, youngest son Kim Jong-un, who is being called the "Great Successor".

Observers fear that because the transfer of power from father to son had not been formalised before Mr Kim's death, it could trigger instability. Regional neighbours are keenly watching events in the internationally-isolated nuclear-armed state.

'Flexibility'

South Korea, which remains technically at war with its northern neighbour following the 1950-1953 Korean War, put its military on a state of alert after Mr Kim's death was announced.

But it has also scrapped a plan to turn on controversial Christmas lights on the border that anger North Korea, and offered "sympathies" over the death.

"The measures we have taken so far are basically aimed at showing North Korea we are not hostile toward the North," Yonhap news agency quoted the president as saying on Thursday.

"An early stabilisation of North Korea's system is in the interests of neighbouring countries," he said. "On future relations with North Korea, there is room for exercising as much flexibility as possible. We will discuss the matter with all political parties."

Ties between the two Koreas have been very tense in recent months, following the sinking of a South Korean warship in March 2010 and the shelling of a border island in November 2010.

North Korea denies a role in the first incident and says Seoul provoked the second.

Relations have also been hit by Mr Lee's refusal to offer aid to Pyongyang without progress on the nuclear issue.

South Korea's nuclear envoy, Lim Sung-nam, is heading to Beijing to discuss how stalled six-nation talks aimed at denuclearising North Korea should be tackled in the wake of Mr Kim's death.

South Korean media have also accused Beijing of failing to communicate with Seoul since the announcement came.