China, the main patron of North Korea, has warned that both steps announced on Friday could lead a Japanese military revival and disrupt the world's strategic balance.

Despite widely-criticised flaws in the missile defense system, designed to shoot down ballistic missiles from North Korea or elsewhere, it is seen as a major step forward for Japan to protect itself in the long-term.

The announcement came as Tokyo finally decided to send its first air force unit to Iraq to prepare for Japanese ground troops after months of foot dragging over safety concerns and in the face of public hostility. The mission is seen as vital to keep up the US-Japan alliance for any future event in Asia.

Although aimed at humanitarian and reconstruction work, it is the first Japanese military deployment since World War II in a country where fighting is still under way, clashing with Tokyo's post-war constitution which bans the use of force in settling international disputes.

Prime Minister Junichiro Koizumi has described Japan's support for US-led efforts in Iraq as imperative for the Pacific alliance. He has also been strident in his defence of the missile defense (MD) system.

"We are not going to wage a war," he told reporters. "But we need to give a message that our country is all right with its own defences and will not to give in to threats."

China's Defense Minister Cao Gangchuan has told his Japanese counterpart Shigeru Ishiba that the MD disrupts the strategic balance in the world and "might promote an arms race".

The official Chinese news agency Xinhua has claimed that the planned troop dispatch will show that "Japan is a major nation that has not only economic power but also military capabilities."

Takashi Inoguchi, a professor of international politics at Tokyo University, admitted that the MD has yet to be proven effective. "But it demonstrates Japan's will to have a full deterrence in place," he said.

The missile shield was also defended by Akio Watanabe, director of Tokyo's Research Institute for Peace and Security. "Recent tests have allegedly proven the system's practical value to some extent," he said.

The government will spend as much as one trillion yen (9.3 billion yen) yen in four years to buy the US-developed system.

It combines SM-3 guided missiles launched by Aegis destroyers to intercept incoming missiles, and Patriot Advanced Capability-3 (PAC-3) missiles to shoot down any missiles that get through from the ground.

The deal was done while Tokyo and Washington continued joint reseach on a still more sophisticated anti-missile system which started after North Korea fired a suspected ballistic missile over Japan and into the Pacific in 1998.

The United States is hurrying to create a missile defence network in the Asia and Pacific region by deploying MDs in South Korea and Australia, the influential newspaper Asahi Shimbun said Friday.

However the project has had its problems. "It will inevitably provoke debate on whether it is worth spending an excessive amount of funds on it," the newspaper said.

Watanabe said the missile shield would be essential for Japanese forces which had shifted their attention since the September 11 terror attack on the United States in 2001 from conventional invasions to such "new threats" as ballistic missiles and terror attacks.

"Japan also wants to join the US-led circle of cooperation in Iraq because its security ties with the United States are important in dealing with the problem at its own door," he added, referring to North Korea's suspected nuclear arsenal.

The missile shield runs the risk of breaching the no-war constitution which rules out "collective defence," or Japan's help for its ally under military attack, said Tokyo University professor Yoshinobu Yamamoto. "Legal amendments will be necessary," he said.

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