Prime Minister Yoshihiko Noda's cabinet Friday gave the green light to shoot down a North Korean rocket if it threatens Japan's territory, as the planned launch raises global alarm bells.

Pyongyang has said it will fire a rocket to put a satellite into orbit between April 12 and 16 to mark the 100th anniversary of the birth of founding president Kim Il-Sung, saying the launch was for peaceful purposes.

But the United States and its allies suspect it is a disguised missile test, and say the launch would contravene UN sanctions aimed at curbing North Korea's missile programme.

"(The cabinet) at a security meeting this morning confirmed the policy to issue a destroy order," an official in the prime minister's office told AFP on condition of anonymity.

The order gave Defence Minister Naoki Tanaka the power to have the projectile shot down, as Japan grows increasingly worried that the rocket may pass over -- or fall into -- its territory.

Last week, Tanaka said he was readying Japan's missile defence systems to destroy the rocket if necessary.

"With this (destroy) order, we will do everything we can to prepare for a fall" of North Korea's rocket, Tanaka told reporters Friday.

The defence chief reiterated that surface-to-air interceptors would be deployed on the southern island chain of Okinawa, below the rocket's forecast flight path, and in central Tokyo, one of the world's biggest cities.

Aegis destroyers equipped with missile defence systems will also be deployed in waters near Okinawa and in the Sea of Japan, he said.

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 also said was aimed at putting a satellite into orbit, passed over Japanese territory without incident or any attempt to shoot it down.

Japan's move come as satellite images showed North Korea has begun preparing for a rocket launch next month despite international condemnation.

North Korea also test-fired two short-range missiles off its west coast this week, South Korean media reported Friday.

Hiroyasu Akutsu, professor at the National Institute for Defence Studies (NIDS), said North Korea is "confident that China will ultimately support them if they launch the satellite" despite the international pressure.

In a study published Friday, NIDS warned improvements in nuclear technology, coupled with the change in leadership that has seen the untested Kim Jong-Un take control in Pyongyang, has increased the risk of a conflict engulfing the region.