North Korea's neighbours responded warily to news of Kim Jong-Il's death Monday, as the prospect of internal transition added another layer of unpredictability to the maverick nuclear-armed regime.

South Korea put its military on alert as news of Kim's death emerged from the hermit state, and Japan, which has also been the object of Pyongyang's aggression in the past, held an emergency cabinet security meeting.

The succession of Kim's son Jong-Un appears to have been well planned and an immediate power struggle is unlikely, analysts believe, but they warn he may want to assert himself with military provocations at some point.

Beijing -- Pyongyang's oft-exasperated ally, which fears regime collapse could drive millions of refugees across its borders -- offered its condolences on the news that Kim Jong-Il had suffered a fatal heart attack.

Some European states, meanwhile, voiced hope that the passing of the "Dear Leader" could be a turning point for the isolated and poor Stalinist country.

Seoul was quick to contact Washington, which has 28,500 troops stationed in South Korea, as it moved to reassert the security alliance that it has long relied on to contain the North.

US President Barack Obama called his close friend President Lee Myung-Bak about the demise of the North's volatile 69-year-old leader.

"The president reaffirmed the United States' strong commitment to the stability of the Korean peninsula and the security of our close ally, the Republic of Korea," the White House said.

US Defense Secretary Leon Panetta and his South Korean counterpart Kim Kwan-Jin in a phone talk "agreed that it is critical to remain prudent with respect to all matters related to our security posture there".

United Nations leader Ban Ki-moon, a former foreign minister for the South, extended his "sympathy" to the people of North Korea "at this time of their national mourning" and vowed to step up help to alleviate its food crisis.

The UN General Assembly, meanwhile, passed a vote that raised "very serious concern" over torture and "inhuman conditions of detention, public executions, extra-judicial and arbitrary detention" in North Korea.

North Korea test-fired a short-range missile off its east coast on Monday, South Korea's Yonhap news agency reported, although it also quoted an unnamed government official as saying the launch was unrelated to Kim's death.

In Seoul, a defence ministry spokesman said: "All commanders are on alert and the South and US are beefing up the sharing of military intelligence. There have been no particular moves by the North's military yet."

Tokyo offered rare condolences on the "passing" of the much-reviled Kim, adding that it hoped there would be no "adverse impact on the peace and stability of the Korean peninsula".

Prime Minister Yoshihiko Noda said he had ordered officials to boost intelligence-gathering on North Korea, to work closely with the US, China and South Korea, and to prepare for further unexpected developments.

Around 50,000 US troops are stationed in Japan, which has no diplomatic relations with Pyongyang and occasionally trades blows with the isolated state over issues including the unresolved kidnap of a number of Japanese nationals.

In China, a foreign ministry spokesman said: "We are shocked to learn that DPRK top leader comrade Kim Jong-Il passed away and we hereby express our deep condolences on his demise and send sincere regards to the DPRK people".

Analysts said Kim's death would be a source of concern to stability obsessed leaders in Beijing who would worry that his young heir has not had enough time to secure control over the government and military.

China is likely to come under international pressure to use its influence to dissuade the regime from carrying out more military provocations like those that rattled Asia last year when the North shelled a border island.

Russian President Dmitry Medvedev -- who met Kim in Siberia in August -- also sent his condolences, a Kremlin spokesman said. News agencies reported that Medvedev had sent the telegram to Kim Jong-Un.

Further afield, EU president Herman Van Rompuy said "the EU is monitoring the situation closely and we count on the future Korean leadership to... commit to peace and stability on the Korean peninsula".

Britain expressed its hope that Kim's death might prove a "turning point".

"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," said Foreign Secretary William Hague.

France's Foreign Minister Alain Juppe said Paris was watching events, "hoping that one day the people of North Korea will be able to find freedom".

Germany, itself divided until 1990, also saw "a chance for things to change", stressing that North Korea must first give up its nuclear programme and improve the "catastrophic social situation of its own people".

Italy, the first G7 nation to establish diplomatic ties with North Korea, said it hoped "a new institutional phase can open as quickly as possible".

Venezuela's left-wing President Hugo Chavez, meanwhile, offered his "most sincere sympathies" and expressed "confidence in the ability of Koreans to conduct their own future to prosperity and peace".

The Catholic Church in South Korea said it hoped Kim's death "could be a sign that the Lord wants a fundamental change in the country".

Scenarios as North Korea faces succession challenge
Seoul (AFP) Dec 19, 2011 - The shock announcement Monday of North Korean leader Kim Jong-Il's death has raised fears of turbulence in the nuclear-armed nation which has been plunged into a second dynastic succession.

Kim's youngest son Kim Jong-Un has been declared in state media as the nation's next leader but little is known about him or his path to power.

Following are some scenarios of how events might unfold.

WHAT WILL HAPPEN NOW?

Kim Jong-Il's body is to lie in state in Kumsusan palace where the embalmed body of his father -- Kim Il-Sung, the founder and "eternal leader" of North Korea who died in 1994 -- is on display.

After Kim's funeral on December 28, thoughts will turn to the succession but there are few details as to how it will be carried out. Kim Jong-Un's status as heir apparent was only made clear in September 2010.

Even Kim Jong-Il, who was openly groomed for the leadership and designated as successor some 14 years before his own father died, did not formally take over the leadership of the ruling party for three years afterwards.

WHO IS IN CHARGE?

The workings of the North Korean leadership -- which managed to keep the leader's death a secret for two days -- are notoriously opaque.

But all the signs so far are that Kim Jong-Un is being installed as the nation's new leader under the guidance of his aunt and uncle who will act as his mentors and as a backstop for a young man with no power base of his own.

Jang Song-Thaek, husband of Kim Jong-Il's only sibling Kim Kyong-Hui, expanded his influence rapidly after the leader suffered a stroke in 2008, forcing the succession plans to be accelerated.

WHAT ARE THE RISKS?

North Korea has a track record of erratic and aggressive behaviour which has alarmed its neighbours and the international community.

Some observers are fearful that the young leader, who has little public profile and no military experience despite being made a four-star general last year, may show his mettle with nuclear tests or military provocations.

Relations with South Korea have been icy since two deadly border incidents blamed on the North last year, which were rumoured to be linked to a show of force by Kim as he tried to ensure his son's succession.

COULD NORTH KOREA IMPLODE?

Many experts believe that North Korea's elites have a powerful vested interest in maintaining the status quo, and that they are not likely to rock the boat unless they have to.

Analysts say turbulence is unlikely, at least in the short term. However, if Kim Jong-Un fails and a messy power struggle ensues, North Korea heads into uncharted territory.

In the meantime, the new leader, who comes into the job with a host of challenges including severe food shortages in a nation which has seen famine in the past, is not expected to adopt an ambitious agenda.

WHY DOES NORTH KOREA MATTER?

North Korea is a formidable enemy, with some 1.19 million men under arms, as well as an arsenal of chemical and conventional weapons including thousands of short- and medium-range missiles.

It has also test-launched Taepodong missiles in its quest for an intercontinental ballistic missile that could potentially strike the United States.

Most estimates say the North has enough plutonium to build six to eight atomic weapons, but it is unclear whether it can make nuclear warheads for its missiles.

Efforts to denuclearise North Korea through six-nation talks including the US and China have dominated regional diplomacy in recent years.

WHAT WILL OTHER COUNTRIES DO?

The North's main ally China is expected to do its utmost to shore up its isolated neighbour, amid fears Jong-Un has not had enough time to cement control over the country's government and military.

Beijing is worried that if the North Korea regime were to collapse, China could be flooded with millions of refugees.

Japan and South Korea's nervousness over the stunning news was reflected as Seoul put its military on alert and the government in Tokyo held an emergency security meeting.

The United States made little immediate comment, but swiftly closed ranks with its ally South Korea, with President Barack Obama calling his close friend President Lee Myung-Bak to discuss the development.