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.

"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.

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.

Amid mounting tensions, North Korea's main ally China urged that "all parties should keep calm and exercise restraint", while a special adviser to US President Barack Obama warned any launch will generate a "strong response."

"If they go ahead anyway, we will want to work with our allies and partners for a strong response," Gary Samore, arms control coordinator at the National Security Council, told South Korea's Yonhap news agency in an interview.

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

The North's atomic programme is expected to be the subject of discussion at the talks on Monday and Tuesday, which are also to be attended by the leaders of China, Japan and Russia.

Leading North Korean newspaper Rodong Sinmun on Friday again blasted the upcoming summit as a "burlesque" and part of a South Korean smear campaign.

It said Seoul's rulers hope to use the event to escalate the "nuclear racket" against the North and moves for a war against it.

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.

Japan's surface-to-air interceptors would reportedly be deployed on the southern island chain of Okinawa, over which Tokyo said the projectile may pass.

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.

North Korea insists it has the right to conduct what it calls a peaceful satellite launch.

But Ban, a South Korean, said the rocket flight 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 its own 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 tonnes of food aid to the North, 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 and nuclear tests, and 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".

South Korea and Japan's nuclear envoys held talks in Seoul Friday over possible steps to take if the launch goes ahead, Yonhap news agency quoted a South Korean foreign ministry official as saying.

In 2009 Japan ordered 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.

That 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.