North Korea's young new leader has shown a confident face to the world, despite a failed rocket launch that was a costly international embarrassment and deprived the nation of much-needed US food aid.

Kim Jong-Un gave an assured 20-minute address Sunday, his first public speech since taking charge of the impoverished but nuclear-armed nation four months ago.

The surprise address came at an elaborate mass military parade marking the centenary of the birth of his grandfather Kim Il-Sung, founder of the dynasty which has ruled the communist nation since it came into being in 1948.

Jong-Un smiled and chatted with military leaders and waved to the crowd from a balcony decorated with giant portraits of his father Kim Jong-Il and his grandfather.

The North also invited an unprecedented number of foreign journalists to report on preparations for the satellite launch and the anniversary celebrations.

They were told to surrender mobile phones and escorted everywhere they went. But officials allowed them unrestricted Internet and phone access from a specially equipped modern media centre.

When the rocket disintegrated last Friday, official media -- unlike after previous satellite launch flops in 1998 and 2009 -- admitted the exercise had failed.

The announcement, albeit brief, was carried on the state news agency and TV news in what was widely seen as an unprecedentedly open response by the secretive state.

After the launch, which went ahead in defiance of stern warnings from Western nations, the United States confirmed it would cancel a recent deal to send food aid in return for a moratorium on missile launches.

Jong-Un, aged in his late 20s, was thrust into the leadership unexpectedly early when his father died of a heart attack last December and must now prove his credentials.

"I think the speech was designed to send a clear signal to the North Korean people and the world that he is firmly in control," said Peter Beck, Korea representative for the Asia Foundation.

"Only time will tell whether that is actually the case," he told AFP.

Beck said Sunday's speech "does help compensate for the embarrassment the leadership must have felt after the rocket launch".

Jong-Un praised his late grandfather for defeating "the most ferocious two imperialisms in one generation" -- a reference to Japan's occupation of Korea and the 1950-53 Korean War, in which Pyongyang claims it triumphed over Washington.

He vowed to strengthen the military "in every possible way" and praised his nation's nuclear programme, saying enemies could no longer threaten his country with atomic bombs.

South Korean observers said it was the North's biggest display of weapons -- some 880 -- in a military parade, including a missile the North appeared to be displaying for the first time.

Jong-Un has a more outgoing image than his father, who spoke just once -- a single sentence -- at a major public occasion. He has been seen hugging soldiers, posing for photos with troops and linking arms with women.

He physically resembles his grandfather, who still commands respect among some North Koreans for his past as an anti-Japanese guerrilla fighter and national founder.

"Jong-Un's style is clearly different from his father and is similar to his grandfather," said Yang Moo-Jin of Seoul's University of North Korean Studies.

"He is quick in decision-making and especially is more media-friendly, probably because of his age," Yang told AFP, adding that the quick admission of the launch failure showed him well informed about international opinion.

"He appears to be more open than his father and is trying to follow in the footsteps of his grandfather, who enjoyed genuine public support."

The analyst said the launch failure was an apparent embarrassment but that Jong-Un had moved in accordance with a well-planned political schedule aimed at strengthening unity.

"His confidence means the transfer of power has been completed and Jong-Un is firmly in power. He is also trying to show the world that the regime is stable," Yang said.