The Prime Minister's Official Residence, the Imperial Palace and other key facilities have been allocated top-priority status for protection under a prospective missile defense program, according to an operational plan made available Wednesday.

The plan envisages joint operations between the Self-Defense Forces and U.S. military forces in Japan for intercepting missiles in the event of an attack on Japan.

The Defense Agency would also consider undertaking joint combat action with the United States to counter an attack even when the target is ambiguous, while taking relevant legal issues into account, according to the plan.

This is likely to spur debate on whether such action would constitute the exercising of Japan's right to collective defense, or coming to an ally's aid militarily.

The government interprets the pacifist Constitution as prohibiting it from exercising this right, even though it is entitled to do so under international law.

The Defense Agency compiled the operational plan to prepare for the deployment of equipment for a missile defense shield starting in March 2007.

Japan and the United States started working toward a missile defense system after North Korea fired a long-range missile over Japan into the Pacific Ocean in August 1998.

North Korea claimed that the missile was a rocket designed to place a satellite into orbit.

The two sides have agreed to move activity on a sea-based missile interceptor from the research phase to the development stage in fiscal 2006; Japan plans to begin deploying components of the missile defense shield by March 2007.

Under the operational plan, the Diet and central government offices are also listed as top-priority facilities for protection, while nuclear power plants and the command facilities of the SDF and U.S. forces in Japan are designated as priority facilities.

But the plan states that the prioritized facilities have not been finalized because the issue requires further examination.

Japan will consult with the U.S. before compiling a joint list of U.S. facilities in Japan to be subject to protection by missiles fired from Aegis-equipped naval vessels or by Patriot Advanced Capability 3 surface-to-air guided missiles.