North Korea's new leader Kim Jong-Un delivered his first public speech Sunday and vowed to push for a stronger military as his country unveiled an apparently new missile.

Kim addressed cheering troops and citizens waving flowers at a major military parade marking the centenary of the birth of his grandfather and the nation's founder Kim Il-Sung.

The parade came just two days after the North's satellite launch fizzled out embarrassingly, when the rocket apparently exploded within minutes of blastoff.

But Jong-Un, aged in his late 20s and in power for less than four months, appeared confident as he oversaw the parade featuring rockets, artillery, tanks and thousands of goose-stepping troops.

"We must strengthen our military in every possible way... and accomplish the goal of building a powerful and prosperous socialist state," he told troops and civilians packing the central square named after his grandfather.

"The time when the enemy threatens and blackmails us with atomic bombs has gone for good," Jong-Un said in reference to the North's nuclear weapons programme touted as one of the greatest achievements of the family dynasty which has ruled since 1948.

"Let's move on towards our final victory!" he said, gesturing at cheering troops who repeatedly chanted "Mansei!" (Hurray).

Jong-Un, clad in his customary dark Mao suit, pledged to improve the lives of people in a nation beset by acute food shortages, an ailing economy and severe power outages.

The ruling party, he said, was determined that North Koreans, "who have endured so many challenges and faithfully served the party, will no longer have to tighten their belts and will fully enjoy socialist prosperity".

Critics say the North's massive military spending could feed millions of malnourished people living outside the showpiece capital Pyongyang.

The US State Department estimates that up to a quarter of the North's gross national product is spent on the 1.2-million-strong military.

Washington has scrapped plans to deliver 240,000 tonnes of food aid after the launch, widely seen overseas as a disguised ballistic missile test in violation of UN resolutions.

One of several missiles on display Sunday appeared new, analysts said.

Ham Hyeong-Pil of South Korea's Korea Institute for Defense Analyses said the weapon -- apparently longer than the North's existing Musudan mid-range missile -- seemed to be a new long-range missile.

"The Musudan, about 12 metres long, is believed to have a range of 3,000 to 4,000 kilometres. But this one appears capable of reaching at least 1,000 kilometres further," Ham told AFP.

"It is certain that the North has developed a new long-range missile."

Christian Lardier, a specialist with the French magazine Air and Cosmos who watched the parade, told AFP it was a Taepodong-class missile about 20 metres (66 feet) long and the first stage was identical to that of the Unha rocket fired Friday.

The parade also featured thousands of troops from a variety of units, trucks with multiple rocket launchers, tanks, armoured personnel carriers, artillery and an array of short- and medium-range missiles.

Five aircraft staged a fly-past.

A sea of people using red, yellow and white paper flowers formed giant displays of the names of the Kims, the national and communist party flags and slogans such as "Glory" and "Undefeatable army".

Kim Il-Sung died in 1994 after bequeathing power to his son Kim Jong-Il. The current leader was thrust into the top post unexpectedly early when his own father Jong-Il died of a heart attack last December.

He has since been cementing his grip on power, taking up top-level posts in the ruling party and on the powerful National Defence Commission last week.

The new leader has a more outgoing image than his father. Kim Jong-Il is believed to have spoken just once at a major public occasion during his 17 years in power -- and that was a single sentence.

Jong-Un, smiling and chatting with military leaders, waved and saluted throughout the parade from a balcony decorated with giant portraits of his father and grandfather.

"Kim Jong-Un, unlike his father, appears to seek a new leadership style that emphasises communication and interaction with the public, just like his grandfather did in the past," said Cheong Seong-Chang of South Korea's Sejong Institute.

Sunday's extravaganza ended with a spectacular fireworks and light show on the bank of the capital's broad Taedong River. Thousands gathered to watch.

"Gorgeous fireworks displayed high up in the sky seem to reflect the Korean people's wishes for the eternal life of their leaders," the national news agency said.