News of North Korea's abortive rocket launch was flashed around the world on Friday but there were four long hours of silence before Pyongyang admitted the highly publicised attempt had failed.

"The earth observation satellite failed to enter its preset orbit. Scientists, technicians and experts are now looking into the cause of the failure," the KCNA official news agency finally said in a terse report.

But there was still no word from officials on the ground for scores of foreign journalists invited by the normally secretive state to witness what was touted as a historic occasion.

The United States and many other nations have condemned the launch as a pretext for testing banned ballistic missile technology.

But the nuclear-armed North, which normally tightly restricts media visits, threw its doors open to emphasise what it called its peaceful intentions in space.

Last Sunday it gave many of them an unprecedented visit to the Tongchang-ri space centre in the country's northwest, where they saw the satellite and the Unha-3 rocket.

On Wednesday they were treated to another ground-breaking visit to the mission control centre in a suburb north of the capital.

During both visits, local media filmed and photographed the journalists.

State television station KRT has frequently broadcast those images, with a commentary saying the foreign reporters were convinced that North Korea would carry out a civilian satellite launch.

But blast-off, first reported by South Korean news outlets, took journalists billeted at Pyongyang's luxury Yanggakdo International Hotel by surprise.

At a specially outfitted media centre in a circular conference room, TV crews and cameramen had set up about a dozen tripods facing a huge white screen which was installed Thursday.

But the centre was almost empty when the news broke from Seoul, and the screen remained blank.

The launch was to have been the centrepiece of mass celebrations marking the 100th anniversary on Sunday of the birth of founding leader Kim Il-Sung.

The impoverished nation, which suffers persistent food and electricity shortages, spent hundreds of millions of dollars on the launch, according to South Korean officials.

The luxury hotel on an island in the Taedong River contrasts with the utilitarian concrete apartment blocks which make up much of the city.

A restaurant on the 47th floor offers a panoramic night-time view -- of a largely darkened metropolis.

In a nation whose leaders are shrouded in a personality cult, the hotel bookshop offers only the works of Kim Il-Sung and his successor Kim Jong-Il, textbooks on the Kimjongilia national flower and similar laudatory material.

Launch failures may be embarrassing, but they are not uncommon even for wealthy and technologically advanced nations.

Christian Lardier, space editor at France's Air and Cosmos magazine, estimated there were an average 75 satellite launch attempts every year worldwide.

Each year there were four or five failures, he said in Pyongyang.

But the North, other analysts said, was likely to be chastened by the failure given its extensive publicity build-up.

"Obviously the rocket launch is pretty embarrassing for Kim Jung-Un and North Korea," said Tate Nurkin, managing director at leading defence publication IHS Jane's.

Kim Jong-Un, grandson of Kim Il-Sung, is working to bolster his authority after taking over power when his own father Kim Jong-Il died last December.

Given the advance publicity "it is hard to imagine a greater humiliation", wrote North Korea expert Marcus Noland on the blog of the Peterson Institute for International Economics.

"Some of the scientists and engineers associated with the launch are likely facing death or the gulag as scapegoats for this embarrassment."