The United States and Japan plan to study a joint missile defense system with Australia to counter growing threats in the Asia-Pacific region, a Japanese government source said yesterday.

Tokyo and Washington have already begun installing a missile shield in and around Japan to ward off potential threats from the Democratic People's Republic of Korea (DPRK). The government official said the new plan also took into account the possibility of a Chinese missile threat against Australia and Guam.

Officials from Japan, the United States and Australia agreed to study the plan when they met in Tokyo in April, the source said.

"The basic tenet of the plan is to step up cooperation between the three countries in the field of security in the Asia-Pacific region," said the source, who declined to be identified.

The source declined to say whether Australia would go ahead and introduce a missile defence system.

"But Australia appears quite willing to contribute to maintaining security in the region, and the joint study program was worked out to deal with the changing security situation in the region," he said.

The United States and Japan started planning a missile defense system after the DPRK test-fired a ballistic missile, part of which flew over Japan and landed in the Pacific Ocean, in 1998.

Japan speeded up implementation of its missile defence program after the DPRK fired a volley of missiles last July - trucking its first PAC-3 interceptors into a military base north of Tokyo in March.

The United States has PAC-3 land-based interceptors on the southern island of Okinawa and SM-3 ship-to-air interceptors stationed at the Japanese port of Yokosuka.

Officials from the three countries are expected to meet and discuss the project later this year, the source said.

In March, Japan and Australia signed a ground-breaking defense pact that the leaders of both countries stressed was not aimed at reining in China.