ANCHORAGE, Alaska — A limited U.S. national missile-defence system is not expected to be activated until at least early 2005, missing President George W. Bush's year-end goal, a military spokesman said Friday.

The announcement came two days after a test interceptor missile failed. But Michael Kucharek, a spokesman with U.S. Northern Command, said the test failure was not the reason for the delayed activation of a few interceptors in Alaska and California.

Rather, he said, military commanders are still in the midst of exercises and demonstrations to make sure the system's elements are functioning and integrated.

In the failed test, an "unknown anomaly'' led to an automatic shutdown of an experimental interceptor missile early Wednesday before it was to launch from the Ronald Reagan Test Site at Kwajalein Atoll in the central Pacific Ocean. A target missile fired from Alaska's Kodiak Island 16 minutes earlier crashed into the ocean.

In previous testing, which critics derided as highly scripted, missile interceptors went five-for-eight in hitting target missiles.

This week's test failure is under investigation by the Pentagon's Missile Defense Agency and contractors developing the ground-based aspect of the system, said agency spokesman Rick Lehner. He said the $85-million test has not been rescheduled.

Lehner also said the tests have no impact on when the first round of interceptors will be powered up at Fort Greely, Alaska, and Vandenberg Air Force Base, Calif.

The missed activation deadline was no surprise to Philip Coyle, a former chief of testing for the Pentagon and an outspoken critic of the Bush administration's planned multibillion-dollar system.

Tests are conducted from coastal platforms in highly controlled settings, using information "no enemy would ever give you,'' Coyle said. Also, satellite-tracking systems and other crucial equipment remain years away from reality, he said.

"Even if the test had been successful, if everything had worked perfectly, I never would have understood how they could declare it operational any time soon,'' he said.

"Too many of the major pieces are missing.''

As envisioned by the military, interceptors will be tied to satellites, ground- and sea-based radar, computers and command centres. Officials said the network is designed to detect and track ballistic missiles, triggering a response by interceptors topped with optical sensors called "kill vehicles'' to destroy enemy warheads aimed at any of the 50 states or Canada.