Japan on Friday readied its missile defence systems to shoot down a North Korean rocket if it threatens the country, as the UN chief warned that next month's launch could jeopardise food aid.

The nuclear-armed North has announced it will launch a rocket in mid-April to put a satellite into orbit, a move that the United States, South Korea and other nations see as a pretext for a long-range missile test banned by the UN.

The move by North Korea's new leadership has set off alarm bells across the region. The Philippines is calling for help from the United States to monitor the rocket, part of which is expected to land off the archipelago.

The preparations by Japan, regularly the target of North Korean barbs, come as world leaders including US President Barack Obama prepare to meet in Seoul early next week for a summit officially focused on nuclear terrorism.

But the North's atomic programme is expected to be the subject of intense discussion at the talks, which are also to be attended by the presidents of China and Russia.

"I have ordered officials to prepare to deploy the PAC-3 and Aegis warships," Japan's Defence Minister Naoki Tanaka told reporters, referring to surface-to-air missiles and destroyers carrying missiles.

"We are talking to relevant local governments about the deployment," he said.

The surface-to-air interceptors would reportedly be deployed on Japan's southern Okinawa island chain, but any order to shoot down the North Korean rocket would first need the approval of Prime Minister Yoshihiko Noda.

Japanese officials have said the projectile may pass over Okinawa.

In a notice to the UN's International Maritime Organisation, North Korea has said the first stage of the rocket will fall in international waters between China and South Korea.

The second stage is expected to splash down just 190 kilometres (118 miles) east of the northern Philippines.

UN Secretary General Ban Ki-moon, who plans to raise the rocket launch at the Seoul summit, said any launch could discourage international aid donors and worsen North Korea's already dire humanitarian situation.

"Such an act would undermine recent positive diplomatic progress and, in its effect on international donors, would likely worsen the humanitarian situation inside the country," he said in a speech in Singapore.

North Korea has warned that any attempts to raise the rocket launch at the Monday-Tuesday nuclear summit in Seoul would be taken as "a declaration of war"

and rejected South Korean demands to call off the launch.

But Ban, a South Korean, said the launch would be a "clear violation" of UN Security Council resolutions and warned that the North already has a "serious humanitarian crisis" on its hands.

The United States voiced doubt last week over whether it could provide food aid to Pyongyang if it followed through on the launch, after an apparent breakthrough deal with North Korea last month.

Washington had said it would deliver 240,000 metric tons of food aid to North Korea, which remains hampered by food shortages after a devastating famine in the 1990s.

In return, Pyongyang agreed to a partial freeze on its nuclear programme, to suspend missile tests and to allow the return of UN atomic inspectors.

The Japanese defence minister, who met with US ambassador John Roos late Thursday, said the two sides "reconfirmed to further strengthen Japan-US cooperation, especially on the North Korean issue".

In 2009, Japan ordered similar missile-defence preparations before Pyongyang's last long-range rocket launch, which brought UN Security Council condemnation and tightened sanctions against the isolated communist state.

The rocket, which North Korea said was also aimed at putting a satellite into orbit, passed over Japanese territory without incident or any attempt to shoot it down. (AFP)