North Korea Tuesday mourned late leader Kim Jong-Il and touted his son and successor Jong-Un as the "pillar of our people" amid international wariness at the upheaval in the nuclear-armed nation.

US President Barack Obama pledged to defend regional allies South Korea and Japan after the reclusive communist state made the shock announcement of Kim's death at the age of 69.

"At the frontline of our revolution stands Comrade Kim Jong-Un, the great successor of the Juche (self-reliance) revolution and the outstanding leader of the party, military and people," the North's official news agency said.

"Comrade Kim Jong-Un is the unwavering spiritual and ideological pillar of our people."

The North has decreed 13 days of nationwide mourning for Kim Jong-Il, who died on Saturday of a heart attack after succeeding his own father in the 1990s.

The regime kept the death a secret for two days until a tearful TV announcer disclosed it Monday and urged people to rally round his youngest son.

The senior Kim presided over a devastating famine but still found funds to build missiles and nuclear weapons during his 17 years in power.

Despite the nation's hardships, state TV aired footage of near-hysterical North Koreans, young and old alike, pounding the ground in displays of abject grief.

The news agency Tuesday carried fresh reports of grief, saying flags of mourning were flying from military bases, factories, commercial facilities and farms and endless queues of mourners were forming.

Jong-Un, who is in his late 20s, was catapulted into the limelight after his father suffered a stroke in August 2008. Last year he was made a four-star general and given top ruling party posts despite having had no public profile.

Analysts said there would be little turbulence -- at least for now -- since regime members at present have a vested interest in preserving the status quo.

"The Kim Jong-Un era has already started," said Paik Hak-Soon of Seoul's Sejong Institute, with observers predicting the younger Kim will be eased into power under the tutelage of his aunt and her husband.

Obama spoke by telephone to Japan's Prime Minister Yoshihiko Noda following a conversation with South Korean President Lee Myung-Bak soon after the death announcement.

"The president underscored the United States' commitment to the defence of our close allies, including Japan," the White House said in a statement.

"He also conveyed the importance he places on maintaining the stability of the Korean peninsula and the region."

South Korea ordered its troops on alert after Monday's bombshell news but seemed to be taking pains to avoid provoking its neighbour.

Defence Minister Kim Kwan-Jin told parliament he would rethink a plan to display Christmas lights near the tense border. "I will reconsider it because it is not timely in the current situation," Kim said.

The communist North had furiously objected to the displays on three towers, which were to be lit up on Friday, calling it "psychological warfare" by its capitalist neighbour.

Senior officials led by President Lee were later Tuesday to debate whether to offer condolences for Kim, despite two deadly border attacks last year which were blamed on the North.

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, and another 50,000 in Japan.

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

US Secretary of State Hillary Clinton said in a statement her "thoughts and prayers" were with the North Korean people "during these difficult times".

She urged the new leadership to "usher in a new era of peace, prosperity and lasting security" on the peninsula.

In Beijing, President Hu Jintao visited the North Korean embassy to offer his condolences. China is the North's sole major ally and its economic prop.

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

Kim took over after his own father and founding president Kim Il-Sung died in 1994.

In the mid-1990s, he presided over a famine that killed hundreds of thousands of his people. Severe food shortages continue and a third of children are stunted by malnutrition, according to UN estimates.

Kim still found the resources for a nuclear weapons programme that culminated in tests in October 2006 and May 2009. The country is believed to have a plutonium stockpile big enough for six to eight weapons.