North Korea's long-range rocket is on its launch platform, AFP reporters said Sunday, as the regime again insisted it was to send a peaceful satellite and not a missile.

The usually secretive North organised an unprecedented visit for foreign reporters to Tongchang-ri space centre in an effort to show its Unha-3 rocket is not a disguised ballistic missile, as claimed by the US and its allies.

Communist North Korea says it will launch the satellite for peaceful scientific research between April 12 and 16 to mark the 100th anniversary of the birth of founding leader Kim Il-Sung. His birthday was on April 15.

"To say this is a missile test is really nonsense," said Jang Myong-Jin, head of the space centre.

"This launch was planned long ago, on the occasion of the 100th birthday of (late) president Kim Il-Sung. We are not doing it for provocative purposes."

The rocket would propel the Kwangmyongsong-3 (Shining Star) satellite into orbit to observe the earth and collect data on forests and natural resources in impoverished but nuclear-armed North Korea, officials said.

A successful mission would burnish the image of young Kim Jong-Un as he seeks to establish his credentials as a strong leader after taking over from his father and longtime ruler Kim Jong-Il, who died last December.

Washington, Japan and South Korea have all condemned the move, saying any missile would contravene United Nations sanctions aimed at curbing North Korea's missile programme. China, the North's major ally, has urged restraint.

There are also heightened fears that the novice administration of the young Kim could be readying for a third nuclear weapons test.

On Saturday in response to the imminent launch, Japan deployed missile batteries in central Tokyo, and Prime Minister Yoshihiko Noda has given the green light to shoot down the rocket if it threatens Japan's territory.

"We will not tolerate any violation of our national sovereignty. We did not shoot down satellite launches from Japan or South Korea. Why are they threatening us?" Jang said.

But the head of the space centre sought to appease the worries of neighbouring countries.

"We can press the button to destroy the rocket and there is also a device in the launching vehicle which can judge whether it is out of the range and destroy itself if it deviates," he said.

It is the first time North Korea has allowed foreign journalists to go to the new space centre built on the Cholsan peninsula, in the northwest of North Korea, 50 kilometres (30 miles) from the Chinese border.

The journalists, who arrived by a special train, were able to observe the rocket from 50 metres (165 feet). It was painted in white with sky blue lettering. There were no indications when it would blast off.

The rocket is 30 metres high and with a diameter of 2.5 metres.

Reporters were also able to see close-up what officials said was the satellite: a 100-kilo (220 pounds) box with five antennae, covered by solar panels to supply it with electricity.

Responding to questions on how much the space programme was costing North Korea while its population was suffering chronic malnutrition, Jang stressed the importance of technological development for the country.

"No matter how much you are hungry, you have to continue to develop technology, as without it you will become the most under-developed country in the world," he said.

He also added that North Korea was planning to launch much more powerful rockets, with a total weight of 400 tonnes compared to the 91 tonnes for the Unha-3 rocket.

"What we saw today, it's civilian," said French expert Christian Lardier, a member of the International Academy of Astronautics. "But this technology can be used for military ends."

The North, which is believed to have enough plutonium for six to eight bombs, tested atomic weapons in October 2006 and May 2009. Both were held one to three months after missile tests.

Preparations are under way in the northeastern town of Punggye-ri, where the North carried out the two previous nuclear tests, a South Korean official in Seoul told AFP on Sunday on condition of anonymity.

"Recent satellite images led us to conclude the North has been secretly digging a new underground tunnel in the nuclear test site... besides two others where the previous tests were conducted," said the source.

Construction of the new tunnel appears to be nearly complete, he said.

Pyongyang's move to fire a rocket in the next week led the US to suspend a recent deal to offer food aid to the North in return for a freeze on some nuclear and missile activities, drawing an angry reaction from North Korea.

In response to the rocket, Japan has deployed missile batteries in downtown Tokyo and dispatched destroyers.

Patriot missiles were Saturday deployed at the defence ministry in central Tokyo and at two other bases in the region to protect the greater Tokyo area and its population of around 35 million.

The ministry also dispatched three Aegis destroyers carrying interceptor missiles, reportedly to the East China Sea, where it has already deployed Patriot missiles on the southern island chain of Okinawa, beneath the rocket's forecast flight path.

Talks over North Korea's nuclear programme are also at an impasse.

The six-party discussions, which involve the two Koreas, the US, China, Russia and Japan, have been at a standstill since the last meeting in December 2008.

Pyongyang in November 2010 disclosed to visiting US experts an apparently operational enriched uranium plant, which could potentially give the North another way to make atomic weapons.