North Korea's five-day window to launch a rocket opened on Thursday with no confirmed firing, but Asian countries remained on alert as Washington rallied world opinion against the communist state.

The morning timeframe in which North Korea plans to launch its 30-metre (100-foot) rocket came and went with no sign of liftoff from a newly built space centre on the country's northwestern Yellow Sea coast.

But the North says the Unha-3 (Galaxy-3) rocket, ostensibly carrying a satellite payload, could go up any day between now and Monday to coincide with Sunday's centenary of the birth of its founding leader Kim Il-Sung.

North Korea is now led by a third generation of the Kim dynasty in the youthful form of Kim Jong-Un, who has been awarded an array of titles including on Wednesday chairman of the all-powerful Central Military Commission.

Fighter jets were heard roaring across Pyongyang's overcast skies early Thursday as the showcase capital stepped up preparations for mass festivities on Sunday to mark the 100th anniversary.

North Korea says its rocket launch is not a banned missile test and that it has every right to send the satellite up, as it promotes the untested leadership of Jong-Un, who succeeded his late father Kim Jong-Il in December.

Lee Yun-Keol, a high-ranking North Korean defector who now heads a think-tank in Seoul, told AFP that he had obtained Kim Jong-Il's last will and testament, which urged the state to develop weapons of mass destruction.

Excerpts of the will were published by Japanese weekly magazine Shukan Bunshun based on the document provided by Lee, who worked for North Korea's bodyguard bureau, the organisation in charge of protecting the Kim family.

"Keep in mind that constantly developing and keeping nuclear (weapons), long-range missiles and biochemical weapons is the way to keep peace on the Korean peninsula, and never drop your guard," the will purportedly said.

"We have to win the psychological war with the United States. By standing up imposingly as a legitimate nuclear power, we have to weaken American influence in the Korean peninsula and work toward lifting international sanctions to prepare external conditions for economic development," it added.

North Korea says it has invited between 150 and 200 foreign journalists to watch the rocket launch and the weekend commemorations, the largest number of overseas media ever welcomed in to the reclusive state.

A large television screen has been installed at a media centre in Pyongyang, apparently to relay live footage of the rocket blasting off.

North Korea says the rocket will place a satellite in orbit for peaceful research purposes, promoting the Kim dynasty's goal for the malnourished country to attain the rank of a "powerful and prosperous state" this year.

But Western critics see the launch as a thinly veiled ballistic missile test, banned by United Nations resolutions. The United States has suspended a deal agreed in February to give food aid to North Korea.

Japanese Prime Minister Yoshihiko Noda said his country was on full alert, while urging North Korea to show "self-restraint until the last minute".

"But we want to be fully prepared for any possible contingency," Noda said, after ordering the deployment of anti-missile batteries on land and at sea to shoot down the rocket if it threatens Japanese territory.

Like Japan, the Philippines ordered flights to divert to avoid being in the Pacific area where debris from the rocket might fall. The UN's maritime agency has also warned shipping to be on alert.

US Secretary of State Hillary Clinton warned North Korea of UN Security Council action if it goes ahead with the rocket launch.

"If Pyongyang goes forward (with the launch) we will all be back in the Security Council to take further action," Clinton told reporters after consulting with her counterparts from the Group of Eight industrial nations.

"There is no doubt that this (launch) would use ballistic missile technology," she said, urging Pyongyang to refrain from "pursuing a cycle of provocation".

China, which is considered to have the most influence over North Korea, repeated calls for restraint from all sides.

"We hope that parties concerned can bear in mind the larger and long-term interests, stay calm and exercise restraint, and maintain peace and stability of this region," foreign ministry spokesman Liu Weimin said.

Secretary-general of the Association of Southeast Asian Nations Surin Pitsuwan also urged the North to avoid provocation.

"As a member of the ASEAN Regional Forum, North Korea must pay attention to the concerns of other members that are destabilised by such a move," Pitsuwan said.