Experimental interceptor bases in Alaska (search) and California can be made ready to fire at incoming ballistic missiles within minutes or hours, the chief of the U.S. Missile Defense Agency (search) said Wednesday.

The report from Lt. Gen. Henry A. "Trey" Obering III (search) suggests technology of the U.S. system is ready to try to shoot down a limited number of incoming intercontinental ballistic missiles with little warning.

The Bush administration has been loath to declare the missile defense system operational, as it had once hoped to have it by the end of last year. Nevertheless, missile defense officials describe the bases, still considered experimental, as having an operational capability. The system still must switch from a test mode to an operational one before it can be fired.

Whether minutes or hours are required to change modes depends on how they are configured when a decision to go operational is made, Obering said. If the system is in the middle of a major software upgrade, it might take hours, he said.

He declined to be more precise.

To shorten the time, Obering said, officials are installing a system that could be set to an operational mode almost instantaneously. He did not say when that would be completed.

How long it takes to prepare the defenses could be critical if the United States should face a surprise attack from North Korea, which the U.S. government perceives as the most likely near-term ICBM threat. It is not clear, however, whether the North Koreans can put a nuclear warhead on a missile that could reach U.S. territory.

An ICBM launched from North Korea could reach the western United States in a half-hour; so whether a surprise launch could be intercepted with so little notice is not certain. Missile defense officials have said they would expect a strategic warning of the possibility of a missile launch that would give time to ready the system.

The system has mechanical blocks on the eight interceptor missiles — six in silos at Fort Greely, Alaska, and two at Vandenberg Air Force, California — that prevent them from launching. Those can be removed quickly, officials said.

The system uses radars in Alaska, California, at sea and in orbit and command centers elsewhere in the United States.

Whether those defenses could shoot down an incoming ICBM remains an open question. In two recent $85 million (euro63.69 million) tests, the interceptors have failed to get out of the silos.