North Korea has hailed Kim Jong-Il's young son as a "great successor" at the helm of the isolated country after the death of the leader.

Kim Jong-Un besides his father. Photo by AP

Pyongyang urged service personnel and citizens to rally behind Kim's youngest son Jong-Un, who is in his late 20s. Last year, he was made a four-star general and given top ruling party posts despite having had no public profile.

It is the nuclear-armed pariah nation's second dynastic succession, and analysts said there would probably be little turbulence -- at least for now -- in the North, whose unpredictable behaviour has long destabilised the region.

"All party members, military men and the public should faithfully follow the leadership of comrade Kim Jong-Un and protect and further strengthen the unified front of the party, military and the public," said the black-clad television announcer.

The official Korean Central News Agency (KCNA) quoted officials and citizens pledging their allegiance to the Swiss-educated new leader.

It described him as "a prominent thinker-theoretician and peerlessly illustrious commander" thanks to whom "the DPRK is more strikingly displaying its dignity and might as an invincible military power."

A KCNA report in the early hours of Tuesday described grief-stricken North Koreans holding late-night vigils at various statues and monuments to Kim.

South Korea put its military on emergency alert after the death was announced but urged its people to stay calm, and swiftly closed ranks with its close ally the United States.

Seoul summoned a meeting of the National Security Council and President Lee Myung-Bak called an emergency cabinet meeting.

The Joint Chiefs of Staff said it had increased monitoring along the border with US forces in the country but had detected no unusual activity.

North and South Korea have remained technically at war since their three-year conflict ended only in an armistice in 1953. The United States stations 28,500 troops in the South.

Lee and US President Barack Obama were quick to talk by telephone after Kim's death was announced at noon (0300 GMT), officials said.

A White House statement said Obama had reaffirmed Washington's commitment to Seoul and to stability on the Korean peninsula.

China and Russia, both influential players in Pyongyang, sent their condolences and observers said Beijing would beef up its all-important patronage to prevent collapse in the communist North.

There was wariness about North Korea's future under Kim Jong-Un, but Britain, France and Germany voiced hope for a new dawn after a tumultuous year that has seen regimes topple across the Middle East.

Secretary of State Hillary Clinton said the United States wanted better relations with the North Korean people, while UN chief Ban Ki-moon vowed to step up help for the shattered country.

"We both share a common interest in a peaceful and stable transition in North Korea as well as ensuring regional peace and stability," Clinton said after talks with Japan's Foreign Minister Koichiro Gemba.

White House spokesman Jay Carney later said the United States hoped Pyongyang would live up to its obligations to end its nuclear programme.

Japan, Korea's former colonial ruler, offered its condolences and Gemba said it was important to ensure Kim's death did not "negatively affect the peace and stability on the Korean peninsula."

The man known as the "Dear Leader," according to KCNA, "passed away from a great mental and physical strain" at 8:30 am on Saturday (2330 GMT Friday) while travelling by train for a field trip.

It said Kim died of a "severe myocardial infarction along with a heart attack." He had suffered a stroke in August 2008 that triggered an acceleration in the succession plans.

Kim's funeral will be held on December 28 in Pyongyang but no foreign delegations will be invited, KCNA said. National mourning was declared until December 29.

Pyongyang test-fired two short-range missiles off its east coast on Monday, South Korea's Yonhap news agency said, though quoting an unnamed government official as saying it was unrelated to the announcement of Kim's death.