North Korea put the body of its late leader Kim Jong-Il on display in a glass coffin Tuesday and heaped praise on his son and successor, amid world wariness at the transition in the nuclear-armed nation.

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

South Korea's government sent its sympathies to the North Korean people despite frosty relations following two deadly border incidents last year.

And it said it would scrap a plan to display Christmas lights near the tense border because its neighbour is in mourning. North Korea has furiously objected to the plan as "psychological warfare".

In Pyongyang, state television showed a sombre dark-clad Kim Jong-Un, youngest son and successor to his father, viewing the corpse along with other high officials.

An honour guard armed with AK-47s watched over the late Kim, dressed in his trademark khaki tunic and partially covered by a red flag, at Pyongyang's Kumsusan Memorial Palace.

The body of Kim Jong-Il's father, founding president Kim Il-Sung, is on view elsewhere in the palace.

The North has decreed 13 days of nationwide mourning for Kim Jong-Il, who presided over a devastating famine but still found funds to build missiles and nuclear weapons during his 17 years in power.

State media have reported scenes of mass grief following his fatal heart attack on Saturday, which the regime kept secret for two days until a tearful TV announcer revealed it and urged people to rally round his youngest son.

Despite the nation's hardships, state TV aired footage of near-hysterical mourners pounding the ground and the North's official news agency carried extensive fresh lamentations on Tuesday.

The Korean Central News Agency (KCNA) touted the inexperienced Jong-Un as the "pillar of our people".

"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," KCNA said.

It cited North Koreans pledging their loyalty, quoting Pak Chol Yong, Korean People's Army soldier, as saying: "We will devotedly safeguard General Kim Jong Un with arms, closely rallied as one in mind around him."

KCNA said that in the 24 hours after Kim's death was announced, more than five million Pyongyang citizens visited statues and monuments to Kim to mourn his passing. The figure represents more than a fifth of the entire North Korean population.

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.

"This is a turning point for North Korea whose reins have been taken by a new generation," said Cheong Seong-Chang, of Seoul's Sejong Institute think-tank.

But 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.

Observers predict that 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."

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.

Kim Jong-Il presided over a famine in the mid-1990s 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.

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 Arab world.

US Secretary of State Hillary Clinton said her "thoughts and prayers" were with the North Korean people and 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, the North's sole major ally and its economic prop, has given its crucial backing to the young new leader.

"We believe that under the leadership of the Korean Workers' Party and comrade Kim Jong-Un, the DPRK (North Korean) people will unite as one and turn their sorrow into strength," said Foreign Minister Yang Jiechi.

Yang has held phone talks with Clinton and South Korean Foreign Minister Kim Sung-hwan on the importance of ensuring security on the Korean peninsula, said foreign ministry spokesman Liu Weimin.

China's state news agency Xinhua later said Yang had also held phone talks with his Russian counterpart Sergei Lavrov.