The Navy yesterday said it intercepted and destroyed a warhead as it separated from its booster rocket during a test off Hawai'i — the first time a ship at sea has shot down a multistage missile.

The USS Lake Erie, a guided missile cruiser equipped with an enhanced Aegis radar system, detected the missile launched for the exercise from the Pacific Missile Range Facility on Kaua'i at 8:12 a.m.

The crew soon fired an interceptor missile that tracked and knocked out the rocket's warhead six minutes later, about 100 miles above the Pacific.

Rear Adm. Kathleen K. Paige, program director of the Aegis Ballistic Missile Defense system, said the exercise mimicked a real-life attack and response.

"This test is very important and I'll go as far as to say historical," Paige said by telephone from Kaua'i. "It verifies ballistic missile defense is real, that it is available today, operationally, at sea."

The achievement is considered significant because medium- and long-range ballistic missiles typically have at least two stages. Intercepting such missiles after they separate is difficult because sensors must be able to distinguish between the body of the missile and the warhead.

All previous tests of the sea-based missile defense system involved short-range missiles that stay intact.

Paige said the target behaved "on the dynamic end of what was possible" but this didn't faze the system or the Lake Erie crew. She declined to elaborate, saying some details are classified.

The military's ground-based missile defense system in Alaska has successfully intercepted separating targets in five of eight attempts when an interceptor was actually launched.

Yesterday's test marked the sea-based system's sixth successful interception in seven attempts since 2001. A sea-based test failed in 2003 when an interceptor missile missed its target.

Lake Erie sailors were told the missile would be launched during a window of several hours yesterday morning, but were not informed of the exact time.

The crew operated under the scenario that "hostile forces" were preparing to launch a ballistic missile against a friendly nation.

Missile Defense Agency spokesman Chris Taylor said the military would take data from the test to fine-tune future tests and computer programs.

The agency expects to conduct three more sea-based tests next year.

The Lake Erie is equipped with technology that allows it to patrol, detect and track intercontinental ballistic missiles.

It is also equipped with interceptor missiles known as Standard Missile-3.

The Navy has outfitted nine destroyers with the same tracking capability and has installed one other cruiser with the SM-3 interceptor missiles.

Since last year, U.S. 7th Fleet ships with the ICBM tracking technology have been patrolling the Sea of Japan, on the lookout for missiles from North Korea.

Pyongyang shocked Tokyo and other nations when it test-fired a ballistic missile over northern Japan in 1998.

Analysts say North Korea is developing long-range missiles capable of reaching Alaska, Hawai'i or perhaps the U.S. West Coast.