Many experts have cast doubt on the effectiveness of such an interceptor system even when it is fully deployed.

Unnerved by North Korea's 1998 launch of a ballistic missile, part of which flew over Japan and landed in the Pacific Ocean, Tokyo agreed to introduce elements of the U.S. missile defence system, but the installation process is far from complete.

"At this stage, Japan has no effective means of countering a ballistic missile attack," said a spokeswoman at the Defence Agency.

Washington says there is evidence North Korea might test-fire its Taepodong-2 ballistic missile, with a range of up to 4,300 km (2,670 miles), and the U.S. has activated a ground-based interceptor missile defence system in case Pyongyang goes ahead.

Although a successful Taepodong-2 launch would not land in Japan, the country remains unprotected for the time being from shorter-range missiles, such as North Korea's Rodong.

The U.S. plans to deploy a ship fitted with Aegis radar equipment and SM-3 missiles capable of shooting down an incoming medium-range ballistic missile. But the USS Shiloh, which will be sent to the Japanese port of Yokosuka, is not set to arrive until August.

Advanced Patriot missiles (PAC-3s), intended as a second line of defence to intercept ballistic missiles on the downward part of their flight, are also scheduled to be brought to U.S. bases by the end of the year.

Japan has had to cut back military spending in other areas to fund its own missile defence plans, which account for 140 billion yen of this year's 4.81 trillion yen defence budget.

Tokyo plans to fit its four Aegis radar system-equipped warships with SM-3s for tracking and destroying any incoming ballistic missile in the middle phase of flight, outside the earth's atmosphere.

The first of the ships will not be ready until some time in the financial year that starts next April, a Defence Agency spokeswoman said.

Japan's own PAC-3s will not be deployed until early next year, while the full quota of 16 launch units will not be complete until four years later.

"Their introduction of missile defence systems, as far as I can tell, doesn't really have anything to do with defending Japan against missiles," said Robert Karniol, Asia-Pacific editor of Jane's Defence Weekly, adding he was sceptical of the systems' effectiveness.

Involvement in missile defence is a political move enabling Tokyo to maintain its close security relationship with the United States and militarily useful in that it helps the two countries integrate their command and control systems, Karniol said.

But Masatsugu Naya, a security expert at Hitotsubashi University, said missile defence could have a psychological effect even if it could not be relied upon to intercept all incoming missiles.

"The question is whether they can shoot down a large enough percentage to make a launching country reconsider its plans," he said. "From that point of view it is effective."