WASHINGTON -- At least twice a week, the military practices what two years ago would have been nearly unthinkable: shooting down a civilian airliner hijacked by terrorists.

Gen. Ralph E. Eberhart, head of U.S. Northern Command, said Thursday that strong safeguards are in place to prevent an accidental or unwarranted shootdown of a commercial airplane. Commanders, pilots and air defense crews are drilled on those procedures at least twice and as many as four times each week, Eberhart said.

The rules allow for an order to shoot down a civilian plane only if there is no other option to prevent a Sept. 11-style attack on the ground, the general said. There are authentication procedures for such orders to make sure "someone can't just get on the radio and say, 'This is the president, I order you to shoot down that plane,"' Eberhart said.

He declined to elaborate on the new procedures, saying they were classified, and would not disclose who in the chain of command had the power to issue a shootdown order.

Military jets were in the air during the 2001 attacks but were too far away to shoot down the airliners before they struck the World Trade Center and Pentagon.

The new procedures are so strong, Eberhart said, that commanders worry more about missing a chance to prevent an attack with a hijacked plane than they do about mistakenly downing a peaceful jet.

"I would take issue with anyone who would say the men and women in our armed forces are trigger happy," Eberhart said at a meeting with reporters.

"I'm more worried that they would be trigger hesitant than trigger happy. We have long discussions with people to see if they're ready to do this."

Eberhart said he has never heard of a case during training simulations in which a pilot or missile battery operator was hesitant to shoot down a hijacked airliner. Those involved have repeated psychological screening and testing on the procedures to make sure they will follow those rules, Eberhart said.

The Pentagon created Northern Command in the wake of the 2001 terrorist attacks to coordinate military defense of the United States and response to attacks or natural disasters.

Eberhart, a four-star general from the Air Force, said the al-Qaida terrorist network responsible for the attacks two years ago has been seriously damaged.

"Most of the varsity players are gone. In most cases we're dealing with the junior varsity team or the freshman team," Eberhart said.

"But we can't rest on our laurels. We've got to keep the throttle up ... If anything, I think we've bought ourselves some time."

Cruise missiles are among the threats to America that are most troubling, Eberhart said, because effective defenses against them are years away, at best.

Cruise missiles fly fast and low, hugging the ground, unlike intercontinental missiles that fly in a huge arc into space before descending to hit their targets. That means cruise missiles are smaller, easier to conceal, easier to launch and harder to track.

On another issue, that of security at U.S. ports, Eberhart said the military and other federal agencies plan to set up command centers at major ports where "everyone involved in security is in one place in an operations center taking notes," Eberhart said. The first two such centers are in San Diego and Norfolk, Va., he said.

Port security has been a weakness but is improving, Eberhart said.

"We're making more progress than people know, but we still have a long way to go," he said.

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Northern Command: http://www.northcom.mil