Japan has rejected an invitation from North Korea to send observers to a rocket launch that Tokyo and its allies say is a disguised missile test, officials said Tuesday.

"It is inappropriate that any Japanese officials participate in observing the launch," top government spokesman Osamu Fujimura said, confirming Pyongyang had invited observers from Japan Aerospace Exploration Agency (JAXA).

"Japan has asked North Korea not to launch a rocket," he said.

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. It insists the launch is entirely peaceful.

But Tokyo, the United States and their 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 invitation, officially extended by Pyongyang's Korean Committee of Space Technology, was "probably the first of its kind", JAXA spokesman Tetsuya Sakashita said.

He said it was delivered personally by officials from the General Association of Korean Residents, Pyongyang's de facto embassy in Tokyo.

Fujimura said Tuesday that Japan would extend for another year unilateral sanctions on North Korea, including a trade freeze and visa ban which were set to expire next week.

Interactions between the two countries have long been tense because of the communist state's nuclear and missile programmes and the past kidnappings of Japanese nationals.

Japanese Prime Minister Yoshihiko Noda's cabinet Friday gave the green light to shoot down the North Korean rocket if it threatens Japan's territory.

In 2009, Japan also 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.