The Pentagon will keep working on its $15 billion THAAD anti-missile defense system despite five consecutive test failures, Defense Secretary William Cohen said Wednesday.

"We're bound to have failures," Cohen told a Senate defense subcommittee a day after the fifth straight failure of the anti-missile system. Cohen said he didn't have the technical "expertise" to give lawmakers a full explanation of the problems. "Technology can be very complicated," he said.The Theater High-Altitude Area Defense failed to intercept a target ballistic missile during a test at White Sands Missile Range, N.M., rising only 1,053 feet in the air, the Pentagon said. The test lasted only 5.8 seconds.

The missile system, built by Lockheed Martin Corp., is being developed to protect troops in the field from attack by Scud and other short- and medium-range missiles.