The envisaged hardware is a high-yield laser cannon loaded on an aircraft with the aim of destroying ballistic missiles when they enter the booster phase after launch, the Mainichi Shimbun said.

The so-called airborne laser system (ALS) has been developed by the US Air Force in a costly collaboration with several firms including Boeing, the report said.

Since late last year, the United States has been informally requesting Japan's participation in technological research for the project in an attempt to defray some of the costs, the report said.

Tokyo exempted joint missile development with the United States from its longstanding ban on arms exports last month.

Japan and the United States have been engaged in joint technological research on a missile defense shield since 1999, a year after North Korea fired a suspected ballistic missile over Japan into the Pacific.

Under the exemption, Japan would be allowed to export parts of missiles to be built for intercepting incoming ballistic missiles.

The daily said that Japan's participation in ALS research could run counter to its official defense policy under which the country can only use force for defensive purposes under its post-war pacifist constitution.

It is difficult to determine the target of a ballistic missile when it is in the booster stage, the report said.

If the ALS is used to shoot down a ballistic missile which is targeted at a third country and while the missile is still on home territory, then the action could be considered to go against Japan's defense-only policy, the report said.

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