"We achieved intercept," said Cheryl Irwin, a Pentagon spokeswoman.

The interceptor missile was fired from Kwajalein atoll in the Pacific some 20 minutes after a modified intercontinental missile roared off a launch pad at Vandenberg Air Force Base carrying the mock warhead and a balloon decoy into space.

A "kill vehicle" released by the interceptor collided with the warhead at 10:29 pm Washington time (0329 GMT Tuesday), she said.

To hit the warhead, a "kill vehicle" used infrared sensors to find the mock warhead and distinguish it from the decoy.

Maneuvering itself into a collision course, it pulverized the warhead at closing speeds of about 26,000 kilometers per hour (15,000 miles per hour).

The United States has successfully intercepted a long-range missile with a ground-based interceptor missile three times in only five attempts.

An interception on July 14 followed two failed attempts, which had raised questions about the technical feasibility of the system.

Pentagon officials had said that a successful test would clear the way for progressively more realistic tests with more decoys and other counter-measures representing more varied classes of threats.

But they stressed that this test was aimed at gathering knowledge, not determining whether the system was ready to be deployed.

"It's just one in a long line of tests we intend to conduct to test a layered (missile defense) system," said Irwin.

The previous administration had said it would deploy a small number of the interceptors if two successful intercepts could be achieved.

But President George W. Bush's administration, which has changed the name of the system from the National Missile Defense System to the Ground-based Midcourse Defense System, has opted not to deploy the system immediately.

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Instead, it has said it intends to pursue development of a wider range of missile defenses, and is pressing Russia to lift restrictions on development, testing and deployment of such systems contained in the 1972 Anti-Ballistic Missile treaty.

So far, the Russians have refused to set aside the treaty, even with US promises of deep cuts in the US strategic nuclear arsenals.

The ground-based missile defense system has long been controversial, attacked as too costly, technically questionable, and politically provocative.

On Friday, the Union of Concerned Scientists released a report that concluded that the system was still in its infancy and too immature to deploy.

"Hit-to-kill has been demonstrated but not under conditions that are operationally relevant," it said.

The debate over missile defense has been overshadowed by the September 11 terrorist attacks on the United States, in which some 3,700 people were killed when hijacked airliners slammed into the World Trade Center and the Pentagon.

But political support for a missile defense system has grown in the US Congress in recent years because of concern that the United States faces a looming threat from long-range missiles in the hands of growing numbers of potential adversaries.

US Defense Secretary Donald Rumsfeld has argued that the September 11 attacks underscored the need for missile defenses, although some critics contend that terrorists with portable weapons of mass destruction pose a more urgent threat than do missiles whose origin would be immediately detectable.

The Ballistic Missile Defense Organization (BMDO) announced today it has successfully completed a test involving a planned intercept of an intercontinental ballistic missile target.

The test took place over the central Pacific Ocean. A modified Minuteman intercontinental ballistic missile (ICBM) target vehicle was launched from Vandenberg AFB, Calif., at 9:59 p.m. EST, and a prototype interceptor was launched approximately 20 minutes later and 4,800 miles away from the Ronald Reagan Missile Site Kwajalein Atoll in the Republic of the Marshall Islands.

The intercept took place approximately 10 minutes after the interceptor was launched, at an altitude in excess of 140 miles above the earth, and during the midcourse phase of the target warhead's flight. This was the third successful intercept for the Ground-based Midcourse Defense (GMD) Segment, formerly known as National Missile Defense.

The test successfully demonstrated exoatmospheric kill vehicle (EKV) flight performance and "hit to kill" technology to intercept and destroy a long-range ballistic missile target.

In addition to the EKV locating, tracking, and intercepting the target resulting in its destruction using only the body-to-body impact, this test also demonstrated the ability of system elements to work together as an integrated system.

The test involved the successful integrated operation of space and ground-based sensors and radars, as well as the Battle Management, Command Control and Communications (BMC3) function to detect the launch of the target missile, cue an early warning radar to provide more detailed target location data; and integration of a prototype X-Band radar (based at Kwajalein) to provide precise target data to the EKV, which received the target updates from the In-Flight Interceptor Communications Systems (IFICS) at Kwajalein.

The EKV separated from its rocket booster more than 1,400 miles from the target warhead. After separation, it used its on-board infrared and visual sensors, augmented with the X-Band radar data provided by BMC3 via the In-flight Interceptor Communications System, to locate and track the target. Sensors aboard the EKV also successfully selected the target instead of a large balloon, which functioned as a decoy.

Only system-generated data was used for the intercept after the EKV separated from its booster rocket. A C-band transponder aboard the target warhead did not provide any tracking or targeting information to the interceptor after the interceptor was launched.

Tonight's test is a major step in our aggressive test program, and is the third successful intercept in five attempts. We will continue to pursue this testing regime to achieve a layered approach to missile defense, using different architectures to deter the growing threat of ballistic missiles carrying weapons of mass destruction.

Over the next several weeks, government and industry program officials will conduct an extensive analysis of the data received during the flight test to determine whether anomalies or malfunctions occurred during the test, evaluate system performance and determine whether or not all flight test objectives were met.

Since the system is in the developmental phase of design and testing, performance of individual elements and the overall system integration was as important as the actual intercept.