The test, the first of its kind in two years, had already been put off four times due to bad weather at launch sites and, on Sunday, due to the failure of a radio transmitter.

On Wednesday, the target missile with a mock warhead was successfully launched from Kodiak, Alaska at (0545 GMT), but it soared uselessly over the Pacific.

"As the ground-based interceptor at Kwajalein Atoll was preparing to launch approximately 16 minutes later, it was automatically shut down due to an unknown anomaly," the MDA said in a statement.

The atoll is part of the Republic of the Marshall Islands, where the Ronald Reagan Test site is located in the Central Pacific Ocean.

"Program officials will review pre-launch data to determine the cause of the shutdown," the agency said without indicating when the next test firing might take place.

Ideally, the missile test should end in an intercept, but the agency says the chief aim is to gather data in preparation for a fully-fledged intercept attempt next March or April.

In earlier tests, target missiles have been successfully intercepted in five out of eight attempts.

But since the last test in December 2002, flight tests have been delayed or cancelled now seven times, adding to questions about the viability of the system.

Despite the lack of recent flight tests, President George W. Bush is expected to declare the system operational by the end of this year.

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