The Defense Security Cooperation Agency announcement Tuesday noted that Japan already has four AEGIS destroyers operating with SM-2 missiles at sea and that two more AEGIS destroyers are under construction.

The new sale involves nine upgraded SM-3 missiles and related equipment.

"Although comparable weapons are not currently deployed in Northeast Asia, the proposed sale of SM-3 missiles and BMD (ballistic missile defense) upgrades to the AEGIS Weapon System will not significantly alter the existing military balance in the region as the proposed sale enhances only defensive capabilities," the Pentagon said.

The U.S. and Japan have been working together for several years to develop both land-based and ship-based missile defenses for Japan and the surrounding seas.

Tuesday's announcement noted that "Japan already has the upgraded AEGIS Weapon System and SM-3 Block IA Standard missiles in its inventory and will have no difficulty absorbing the additional upgraded weapon system and missiles."

In addition to past sales of U.S. equipment, the two countries signed an agreement in 2005 allowing Japan to produce Patriot Advanced Capability-3, or PAC-3, missiles for deployment during fiscal 2006.

The missiles are designed to intercept and destroy incoming ballistic missiles, cruise missiles and enemy aircraft.