The U.S. has made it clear that the umbrella is not being designed with Russia or China in mind, and should not be seen as affecting their nuclear deterrents.

Beijing, in particular, is not convinced, largely because a theater missile shield defending Taiwan could render Chinese missiles less effective in the event of a future conflict.

"The incorporation of Taiwan into any foreign missile defense system is unacceptable and will seriously undermine regional stability," Beijing and Moscow said in a joint communique back in 2000.

Amid increased tensions across the Taiwan Strait, Chinese premier Wen Jiabao is planning to raise U.S. support for Taiwan during his forthcoming visit to Washington.

The Australian government says it has informed the Chinese and other countries in the region of its decision to back the defense shield.

In a television interview Friday, Foreign Minister Alexander Downer suggested that the Chinese were less hostile to the idea than had been previously the case.

"They've been very moderate in their response," he said.

"I don't think you could say that they've been supportive of this particular proposal, but I think they increasingly understand this isn't directed at China. or isn't designed to intervene in the China-Taiwan issue - which is of course their great area of sensitivity."

A spokesman for the Chinese Embassy in Canberra said Friday there was no official response yet to the decision.

North Korean concerns

The missile defense system aims to detect incoming enemy missiles and then launch missiles from land- or sea-based platforms to intercept and destroy them in mid-air.

The "rogue" state most obviously of concern in the region is North Korea, which in 1998 test-fired a ballistic missile that flew over Japan before landing in the Pacific Ocean.

Pyongyang, which has admitted it has nuclear weapons, is developing longer-range rockets that American officials have warned could target the U.S. West Coast within a couple of years. That would also put Australia within range.

Canberra has not elaborated on the extent of its planned participation, but indicated that it could involve the use of Australian warships, and incorporate a U.S. satellite tracking station in central Australia called Pine Gap.

Defense Minister Robert Hill said he did not believe Australia would host interceptor missiles on its soil.

Likely areas of co-operation could include cooperation to ensure early warning of the launch of an enemy missile; acquisition of ship- and land-based sensors; and science and technology research, development, testing and evaluation.

Hill said in a statement the aim of the system was not to threaten other countries but to discourage them from investing in ballistic missile systems.

"The government is concerned that Australia might one day be threatened by long-range missiles with mass destruction effect and believes that investment in defensive measures is important," he said.

Space ambitions

The missile defense shield under development has been dubbed, mostly by opponents, the "son of Star Wars," in reference to President Reagan's Strategic Defense Initiative (SDI), or "Star Wars" proposals announced 20 years ago.

Although the current plans trace their roots from SDI, they are considerably more modest and Earth-based than the original ones, which envisaged a massive, space-based shield against a potential Soviet nuclear attack.

Critics continue to accuse the Bush administration of wanting to dominate space, however.

"The U.S. is not building a defensive system. It is planning to militarize and control space," said Dr. Hannah Middleton of the Australian Anti-Bases Campaign Coalition Friday. The coalition is opposed to U.S. bases and related facilities on Australian territory.

An organization called the Global Network Against Weapons and Nuclear Power in Space said the missile defense plan "could ultimately carry warfare into the heavens."

Responded to the Australian decision, network coordinator Bruce Gagnon said the U.S. was finding the costs of developing the program so high it was "working overtime to convince allies to help with investments in the research and development phase of the project."

Left-wing politicians in Australia have also slammed the government's decision.