The Pentagon's costly missile defense system failed a fifth attempt to intercept a target Tuesday.

The Theater High-Altitude Area Defense, or THAAD system, built by Lockheed Martin Corp., failed to intercept a target in a flight test at White Sands Missile Range, N.M., the Pentagon said in a statement. It appeared to be caused by a booster rocket problem."Preliminary investigation indicates that the THAAD missile lost control shortly after launch," said the statement.

The missile hit the missile range about two miles north of the launch site. The interceptor missile and target debris landed on the missile range, the statement said. "Analysis of the flight data is under way to determine the cause of the malfunction," the statement added.

The failure is a major blow to the THAAD program, which has been struggling to prove it could be used to defend U.S. troops in the field against missile attack.

"This was the fifth flight test - zero-for-five," said a one Pentagon official, speaking on condition of anonymity.

The THAAD system is designed to provide U.S. forces in the field protection from attack by Scud and other short- and medium-range missiles. Its technology involves "hitting a bullet with a bullet," a technical challenge the program's designers and managers have so far failed to meet.

THAAD is designed to provide broader defensive coverage than the Patriot missile system first used in the 1991 Persian Gulf War. The failure could have implications beyond theater missile defense and affect debate over development of a national missile defense shield.

Impatient with the pace of the Pentagon's efforts to develop a national missile defense system, Senate Republicans are pressing to commit the nation to such a shield even before the technology is fully developed.

The legislation, which already has 50 sponsors in the chamber, drew the strong opposition of the Clinton administration and its Senate allies, who are threatening to block the bill through delaying tactics.

The bill by Sens. Thad Cochran, R-Miss., and Daniel Inouye, D-Hawaii, has wide GOP support. It would direct the Pentagon to deploy such a system as soon as technology permitted.

The administration's present program requires identifying an emerging ballistic missile threat first; then, if necessary, three years would be provided to put the program into effect.

Critics argued that the legislation would commit the United States to deploy a technology that hasn't even been developed yet.

Fifteen years after President Reagan disclosed his hopes for a "Star Wars" missile defense, none has been developed to protect the entire nation, even though nearly $50 billion has been spent toward achieving such a system.

Both Defense Secretary William Cohen and Gen. Henry Shelton, chairman of the Joint Chiefs of Staff, oppose the measure.