Flight tests of the nation's missile defense system (search) will not resume until this fall at the earliest as the military revamps the program following two failures in the past seven months, a military official says.

The military may conduct two tests by year's end, with the earliest possibly this fall, said the official, who spoke on condition of anonymity because no schedule has been announced.

It is uncertain whether the military will have a target missile ready for launch, however, and the first test may not involve an attempt to hit a target.

The delay further protracts Pentagon efforts to validate a multibillion-dollar program that supporters say will help protect the nation from intercontinental ballistic missiles. Critics say that claim remains unproven.

Even though the military occasionally activates interceptor bases in Alaska and California, they are not yet on around-the-clock alert as envisioned. The system has not had a successful intercept of a target since October 2002. Three tests have ended in failure.

The Bush administration had said the system would be working by the end of 2004.

An independent review, performed this year by experts for the Pentagon Missile Defense Agency, suggested that the rush to deploy the defenses led to inadequate quality control during the tests. The report was posted online by the Center for Defense Information, a defense policy think-tank in Washington.

Missile Defense Agency spokesman Rick Lehner acknowledged that the report raised some issues regarding quality control that, "quite frankly, we didn't pay enough attention to, and now we are."

President Bush is seeking $9 billion for the program in the upcoming budget year, $1 billion less than previously planned. Since 1983, the government has spent $92 billion to develop a system to shield the U.S. from attack by ballistic missiles.

In the two most recent tests, each costing $85 million, the interceptors failed to get out of their silos.

Last Dec. 15, the test missile did not launch because of a problem with communications software. The second test, on Feb. 14, failed because an arm that holds up the interceptor did not fully retract in the moments before it launched, officials said. The interceptor shut down automatically.

Both tests were to involve launching an interceptor from Kwajalein Island (search) in the Pacific Ocean at a target launched from Kodiak Island (search), Alaska.

The system was successful in five of eight previous tests in highly scripted attempts to intercept a target missile carrying a mock warhead.

The Missile Defense Agency is putting together a new schedule for future tests, Lehner said. The goal is to make the tests more rigorous for the interceptor missiles and less likely to fail due to problems in test equipment.

Whatever becomes of the testing, the Pentagon will forge ahead this summer with installing 10 new interceptor missiles at its base in Fort Greely, Alaska, officials said. Greely has six interceptors already in place.

Two more interceptors, stationed at Vandenberg Air Force Base in California, are supposed to be able to shoot down several long-range missiles that conceivably could come from North Korea. The lack of successful tests gives some critics little confidence in the system.

"The system in Alaska and California has no demonstrated capability to defend the United States under realistic operational conditions," said Philip Coyle, a former chief of testing for the Pentagon, who criticized the earlier, successful tests as highly scripted.

The agency would need 20 or 30 more developmental tests before it will be ready for realistic testing, Coyle said.

"If it takes two or three years to get a success, at that rate, those 20 or 30 tests could take them 50 years. They obviously need to improve the pace as well as the realism, but they haven't been able to do it," he said.