The Japanese government has decided to move forward with a ballistic missile defense program with the United States, a government official said Saturday.

Chief Cabinet Secretary Shinzo Abe said the government has decided to proceed with the joint development of a missile interceptor for the program, designed to use defensive missiles to destroy attacking ones.

The effort is part of sweeping changes to Japan's defense policy launched by Prime Minister Junichiro Koizumi. Critics say the reforms are dismantling the country's post-World War II policy of pacifism.

A year ago, Japan adopted new defense guidelines that ease its nearly three-decades-old ban on arms exports to allow it to develop a missile defense program with its closest ally, the United States.

North Korea became one of Tokyo's biggest security worries after it test-fired a long-range ballistic missile over Japan in 1998, prompting Tokyo to begin researching missile defense.

Tokyo and Washington have been discussing an estimated $3 billion joint defense shield with Japan's share at about one-third, defense officials said.

Tokyo and Washington reached an agreement in October on the realignment of U.S. forces in Japan. The agreement also provided for the deployment of a missile defense system in the island nation.

The U.S. and Japan have long been working on a joint defense system. Since last year, the U.S. Navy has been patrolling the Sea of Japan, on the lookout for missiles from North Korea.

Japan's Defense Agency said earlier this month that the U.S. and Japanese militaries are looking at a site in northeastern Japan to base a radar system for the missile shield.

Japan's postwar pacifist constitution renounces war and the use of military force in settling international disputes.

Japan has deployed some 600 troops on a humanitarian mission in southern Iraq since early 2004 — its first military dispatch to an active war zone since World War II. Earlier this month, Tokyo extended the troop deployment for one year, defying rising domestic opposition to the mission largely over safety concerns.