Lieutenant General Ronald Kadish, director of the Missile Defense Agency, said no decision had been made on when to put the first interceptor missile on alert.

"From a common sense standpoint, the earliest would come when we have at least one bullet in the chamber, one missile," Kadish told reporters here.

The first five ground-based interceptor missiles will be in their silos at Fort Greely, Alaska by September, complete with hardware, software and communications, he said.

Three or four more missiles will be added in December, and another 10 at both Greely and Vandenberg Air Force Base in California the following year if Congress funds the Pentagon's 9.2-billion dollar budget request, he said.

Critics have charged that the system is being fielded without being sufficiently tested. The General Accounting Office, a congressional watchdog, said in a report this month that the system was "largely unproven" despite eight intercept flight tests.

Two more flight tests -- only one of them an attempted intercept -- are slated to be held in a few months, Kadish said.

Asked whether the system could be put on alert even if those tests fail, Kadish said, "it could."

"When it goes on alert it could be a function of many different things, including the world situation at the time," he said.

But he acknowledged that successful flight tests were "confidence builders" that showed that all the systems parts work together as designed.

"If they both fail, we're got big problems that we're going to have to go figure out what to do about," he said.

The Bush administration has made fielding a missile defense system a top priority, arguing that even a rudimentary system would deter countries from seeking to develop long-range missiles.

Kadish said US intelligence indicates no change in the long-range missile threat from North Korea and Iran.

The first phase of the ground-based system would defend all 50 US states against a missile attack by North Korea, but not Iran.

Of eight previous attempts at intercepting a long-range target missile, five have been successful.

But the last intercept attempt was more than two years ago in December 2002, and the next test is more than three months behind schedule.

The interceptor's "kill vehicle," which is guided into a collision course in space with an incoming missile, has since been redesigned.

Kadish said the discovery of circuit board failure in March prompted an intensive review of the the redesigned kill vehicle, which has set back the testing.

"Once that's complete we'll do the flight test. We expect them to be successful. Let me make that clear," he said. "But the nature of rocketry and the way things are we may have a mishap, and something small could be very catastrophic."

"We're going to do flight tests when we're ready, not on a scheduled date just to do it on a scheduled date, and the preponderance of evidence is going to drive the decision on whether or not to use it on alert," he said.

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