"Japan has only a low awareness on protecting information regarding national security," Defense Agency Director-General Fukushiro Nukaga told reporters after the leak in the 1990s came to light.

The agency said data on developing a new surface-to-air missile system had been "leaked to a software company believed to be linked" to a science group under the General Association of Korean Residents in Japan in the 1990s.

The residents' association, commonly called Chongryon, has been working as North Korea's de facto representative office in Japan because the two countries have no diplomatic ties.

"The Defense Agency extremely regrets the information leakage. We will investigate the matter urgently and take appropriate measures promptly," an agency spokesman said.

"We will know whether the leakage has any impact (on Japan's security) as the investigation proceeds."

The data leaked between 1993 and 1995 concerned the development of a future version of a missile system which Japan's ground forces started deploying in 2003, he said.

The software company obtained the data through Japan's Mitsubishi Electric, which was assigned by the government agency to help improve the mid-range surface-to-air missile system.

The software firm was a subcontractor of Mitsubishi and is believed to have ties to a Chongryon group that promotes interaction between Korean scientists in Japan and their counterparts overseas, the spokesman said.

The case came to light after the Defense Agency got a tip-off from police, he added.

North Korea stunned the world by launching a missile over Japan in 1998, prompting Tokyo to send up a spy satellite and take other security measures.

Japan teamed up with the United States to develop a missile shield after the 1998 launch.

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