US Defense Secretary Donald Rumsfeld said he was on and off the phone with top US commanders almost continuously for days before the missile tests.

"I received the notification of the launch of these missiles probably within of a minute of when they occurred," he told reporters before a meeting with Georgian President Mikhail Saakashvili.

Pentagon officials were circumspect, though, about how the multi-billion dollar missile defense system performed.

"What I will tell you is that each and every launch was detected and monitored, and that interceptors were operational during the missile launches that took place," said Pentagon spokesman Bryan Whitman.

"The commander of NORAD (the North American Air Defense Command) was able to determine rather quickly that the missiles didn't pose a threat to the United States or its territories," he said.

The North Korean Taepodong-2 was a dud, failing within 40 seconds of lifting off from a launch pad in eastern Korea, so no US interceptor missiles were fired, according to defense officials.

The other six medium- and short-range missiles landed in the Sea of Japan.

"The Taepodong-2 is estimated to have the range that could conceivably reach the United States and the fact that it failed is fact, but it does not change the nature of the launch," Rumsfeld said.

It was unclear Wednesday to what extent the integrated US missile defense system was used to detect and track the launches.

A network of early-warning satellites and radars is supposed to feed data to a command center in Colorado Springs, Colorado, where they are processed by powerful computers linked to interceptor missiles in Alaska and California.

Analysts said the United States likely had as many assets focused on North Korea as on the US shuttle launch, which occurred at about the same time as the Taepodong-2 went up.

"I think they had all of their sensors watching very carefully. This is the big one. The Taepodong-2," said John Pike, director of GlobalSecurity.org.

"I think the toss up is whether they had more eyes on the missile or on the space shuttle. They had a lot of metal in the air," he said.

Pike said it may be several days before US analysts get a clearer picture of what happened to the Taepodong-2. The missile's behavior may offer clues to whether the cause of the failure was structural or guidance problems or something else, he said.

Others said initial confusion over the number and types of missiles launched underscore the vulnerabilities of the missile defense system.

"If you don't see all of the missiles an enemy fires, or if they fire too many, even the most futuristic missile defenses we can imagine will be overwhelmed," said Philip Coyle, a former Pentagon weapons evaluator.

The United States has early-warning radars in Clear, Alaska, and Beale Air Force Base in California, as well as a decades-old network of satellites that typically would be used to detect missile launches.

But the missile defense system has a broader constellation of sensors to draw on.

Eleven Aegis warships with Spy-1 satellites have been modified for missile defense tracking missions. Two early-warning radars also have been upgraded to track warheads through space.

And new X-band radars have been developed that are supposedly powerful enough to target an object the size of baseball over California from the US East Coast.

The Missile Defense Agency last month deployed the first sea-based X-band radar off Hawaii and a Forward Based X-band Transportable radar to Shariki, Japan.

A spokesman for the agency said he did not know if they were ready for use in the current situation.

Since the mid-1980s, the United States has spent more than 90 billion dollars to develop a defense against long-range missile attack, a quest that has been fraught with controversy.

The fruit of the effort is what Pentagon officials say is a rudimentary defense against a limited missile attack by North Korea, consisting of nine interceptor missiles at Fort Greely, Alaska, and two more at Vandenberg Air force Base in California.

The interceptor missiles are designed to destroy an incoming warhead in space.

In practice, however, the system has succeeded only in five of 10 attempts to intercept a mock warhead in space. The last intercept occurred in 2002, and that was followed by two failures.

Critics contend that even the successful tests were bogus because they were operationally unrealistic and used surrogates for components of the system that were still in development.

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