A US prototype missile interceptor went lost in space on January 18 night after failing to hit a mock warhead target launched from California, the Defense Department announced.

The failure is an apparent setback in the Pentagon's drive to develop a national missile defense.

"An intercept was not achieved for reasons unknown at this time," Pentagon spokesman Rick Lehner said.

The interceptor rocket was launched from a US Army missile range on Kwajalein Atoll in the Marshall Islands, about 20 minutes after the target missile lifted off from Vandenberg Air Force Basein California at 9:19 p.m. EST (0219 GMT on Wednesday) and headed over the Pacific Ocean.

The mock warhead was meant to simulate a nuclear attack on the United States, and a successful intercept would have provided a dramatic boost to the Pentagon's effort to show missile defense can work.

Lehner said the Pentagon would conduct an extensive review of the test data to determine why the interceptor missed the target.

Defense Department officials said before the test that even if the interceptor failed to hit its target, there were other important goals for the test, including the first test of a computerized battle management system that is designed to communicate with the interceptor as it soars into space.

The first major test of the missile interceptor -- known as a "kill vehicle," or by its Pentagon nickname, "smart rock" -- was last October 2.

The officials said earlier that if Tuesday's test succeeded, it could lead to a decision by US President Bill Clinton this summer to commit the United States officially to building a defense against missile attacks by other countries at a cost of at least 12.7 billion US dollars.

Arms control experts said that the US decision to set up the national defense system will aggravate the arms race, thus affecting world peace and stability. (Xinhua)