Two Air Force generals have been authorized to order the military to shoot down any civilian airliner that appears to be threatening U.S. cities, Pentagon officials said Thursday.

Seeking to reassure America's travelers of their safety, Secretary of Defense Donald H. Rumsfeld said: "There are a lot of safeguards in place."

He said he had crafted the new rules of engagement with Gen. Henry H. Shelton, chairman of the Joint Chiefs of Staff.

Asked at a Pentagon press conference what assurance passengers would have against mistakes, Rumsfeld said:

"The rules of engagement are addressed on a continuing basis with a great deal of care and sensitivity."

Michael Perini, director of public affairs for the North American Aerospace Defense Command, stressed that only under the most extraordinary circumstances could the generals act without having consulted with the president, secretary of defense or other higher-ups — only as a last resort and only if there simply was not enough time to consult.

From NORAD offices in Colorado, Perini said he could not discuss a specific situation in which the regional commanders might take such action because it would reveal too much about military planning for the defense of U.S. airspace.

The move follows revelations by Vice President Dick Cheney that in the hours after the Sept. 11 attacks on the World Trade Center and the Pentagon, President Bush had ordered the military to intercept and shoot down any commercial airliners that refused instructions to turn away from Washington.

After receiving warnings that commercial planes had veered off course, military fighter jets scrambled over Massachusetts and Virginia on the day of the terrorist attacks. But they were unable to respond in time, officials have said.

Maj. Gen. Larry K. Arnold at Tyndall Air Force Base, Fla., would have authority to order the downing of a threatening commercial flight over the 48 contiguous states. Lt. Gen. Norton A. Schwartz at Elmendorf Air Force Base, Alaska, would have authority for Alaska.

Following the terrorists' attacks, Rumsfeld ordered fighter jets at 26 bases nationwide to be prepared to take off on 10-minute notice.

Reserve units have been called up to supplement the effort in which F-15s and F-16s fly 24-hour patrols over dozens of American cities.