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But with just eight interceptor missiles in place in Alaska and California, it would have to be a pretty small attack, said Major Gen. John Holly, who heads the program for the Pentagon's Missile Defense Agency.

While the system has not officially been declared active by the Bush administration, it has an emergency capability that can be switched on at any time, Holly said.

"If directed, we could provide a limited defense against an attack out of Northeast Asia," he said.

Holly briefed Alaska lawmakers on the system Tuesday. Fort Greely is home to six interceptors, and 10 more are expected to be installed by the end of the year. Sites in the Aleutian Islands and Kodiak are also part of the system.

The system's first priority is to shield against potential threats from North Korea, which last year tested a ballistic missile engine with the range to reach Alaska. Later phases include protection against missile threats from the Middle East, and eventually covering troops and allied nations.

Holly told lawmakers that Canada's refusal to participate in the missile defense system will not affect operations in the initial startup phase. He told reporters later that the military did not have plans to place hardware in Canada.

"There is not an impact by the fact that Canada has not elected to participate currently in this particular system," Holly said.

In tests in December and February, interceptors failed to launch to destroy their targets over the Pacific Ocean. Those failures were attributed to a software glitch in the first case and a faulty retracting arm in the second, Holly said.

"Those are very disappointing events. Neither of them dealt with fundamental design issues of the overall system," he said.

In the first test, the launch was aborted when a software program determined systems communications weren't what they should be. Holly said that has been fixed. The cause of the second failure, when an arm used to steady the interceptor in its silo did not fully retract, has not been determined.

Two more tests are scheduled between April and June, Holly said.