Japan on Friday ordered its military to shoot down a North Korean rocket if it threatens the nation's territory as Washington put anti-missile destroyers into position, ramping up pressure on Pyongyang.

The moves came as a US think tank said recent heavy snow could hamper the hermit nation's launch effort.

Tokyo has readied surface-to-air missiles in and around the capital, as well as in Okinawa, and has deployed Aegis warships in neighbouring waters.

Prime Minister Yoshihiko Noda, who is facing an uphill election battle this month, visited the defence ministry in Tokyo where Patriot missile batteries stood ready.

"If the missile is launched, I want you to act calmly and resolutely to secure Japanese people's life, property and safety," he told about 100 Self-Defence Force (SDF) personnel clad in camouflage uniforms.

The United States has mobilised ships equipped with ballistic missile defences in readiness for the rocket launch, the head of US Pacific Command said.

The USS Benfold and the USS Fitzgerald have been sent to the area to "monitor any potential missile launch by North Korea and to reassure regional allies should a launch occur", a US Navy official told AFP in Washington.

The communist North announced last week a December 10 to 22 window, its second long-range rocket launch this year after a much-hyped but botched attempt in April.

Japan readied similar defences last time, eliciting proclamations from Pyongyang that attempts to intercept it would be "an act of war".

North Korea insists -- as it did in April -- it is launching a satellite, but the international community sees it as a poorly disguised test of ballistic missile technology, which is banned under United Nations Security Council resolutions.

Pyongyang-watchers say the launch window is twice the length it was last time, reflecting the difficulties technicians may encounter in the harsh winter weather of the Korean peninsula.

Analysis of fresh satellite imagery suggests preparations at the Sohae satellite launch station are proceeding "more slowly than previously reported", the US-Korea Institute at Johns Hopkins University said.

"Since this is Pyongyang's first attempt to launch a long-range rocket in winter, weather may be a new factor," Nick Hansen, an expert on imagery analysis, wrote on the institute's website 38 North.

Recent images taken on December 4 showed no tracks in heavy snow that had fallen on the launch site the day before, suggesting at least a temporary halt in operations, Hansen said.

This would raise doubts over South Korean media reports on Wednesday -- citing government sources -- that the North had completed installing all three stages of the Unha-3 rocket on the launch pad.

Washington and Seoul have urged Pyongyang to scrap the launch while Tokyo has postponed talks originally planned this week with North Korea.

Tokyo pledged it would move swiftly to warn its citizens once Pyongyang's rocket is airborne, sending alerts to broadcasters and through Twitter and other social media, Chief Cabinet Secretary Osamu Fujimura said.

"We would like people to carry on as normal because the missile won't fall towards Japan if all goes as expected," he added.

UN diplomats inside and outside the Security Council have started consultations behind the scenes on what action to take if Pyongyang goes ahead with the launch, Kyodo News reported.

Japan, the United States and South Korea have agreed to demand the UN Security Council boost sanctions on North Korea to levels that match those on Iran, the Asahi Shimbun daily said.

That would include increasing the list of financial institutions, entities and individuals that are subject to asset freezes, it said.