WASHINGTON — U.S. missile-defence systems may not have faced testing rigorous enough to ensure they would work during an attack, said a congressional study released Friday.

The report, prepared by the General Accounting Office, an investigative arm of Congress, calls for the military to conduct more realistic testing before making the systems operational.

The first of a series of interceptor missiles, designed to shoot down intercontinental ballistic missiles launched by North Korea or other belligerents, are expected to be ready by the end of the year.

A spokesman for the military's Missile Defense Agency disputed aspects of the congressional report. Rick Lehner said the military is fulfilling a congressional mandate to deploy missile defences as soon as it can -- even though the program is still in a research and development phase. That means additional testing will take place after the first interceptors are deployed, he said.

Right now, the military would have little hope of stopping a missile if it were fired at a U.S. city. That could change by the end of the year, military officials said.

"We're providing this capability where we have no capability now," Lehner said.

The U.S. administration has made the deployment of missile defences a key aspect of its national security policy, saying it is vital to defend the country against missiles launched by hostile states. Critics said the technology is immature and too expensive and fails to address the greater threat of weapons of mass destruction being brought into the country by terrorists or other means.

The initial system of long-range interceptors would be placed in Fort Greely, Alaska, and Vandenberg Air Force Base, Calif. A set of shorter-range interceptors will be deployed on navy destroyers.

More advanced missile defences, including a laser cannon mounted on a Boeing 747, are also in development.

Citing figures from the Missile Defense Agency, the GAO report said missile-defence programs will cost $53 billion between 2004 and 2009.

The GAO report said several programs are over-budget or delayed. It also criticized the military's development strategy, saying the program lacks firm milestones and estimates of its total cost over its lifetime.

"As a result, decision-makers in DOD and Congress do not have a full understanding of the overall cost of developing and fielding the Ballistic Missile Defense System and what the system's true capabilities will be," a summary of the GAO report said.

In a statement, U.S. Senator Jack Reed accused the administration of "blindly spending billions of dollars every year" on an untested defence.

"When the administration does deploy a system this fall, the United States will unfortunately still be a long way from an effective defence against real missiles," he said.

Earlier this week, Lt.-Gen. Ronald Kadish, director of the Missile Defense Agency, told a Senate appropriations subcommittee he expects to meet President George W. Bush's goal of having 20 interceptors in place by the end of 2005.

"It's still a major challenge for us over the next six months to do this but right now what I see is we will have up to eight by this calendar year," Kadish said.

He said the system would provide a "capability to defeat near-term threats of greatest concern."