The American F-14s that shot down two Libyan MiG-23 fighters were operating under guidelines that say U.S. forces can open fire only when threatened, administration officials said.

Defense Secretary Frank Carlucci, discussing the incident with reporters Wednesday, said the F-14s from the aircraft carrier USS John F. Kennedy, fired in self-defense and were operating under "normal peacetime rules of engagement."Such rules are never publicly disclosed, except in general terms.

They were the subject of controversy after an Iraqi jet fired at the USS Stark in the Persian Gulf May 17, 1987, killing 37 U.S. sailors, and after the USS Vincennes shot down an Iranian commerical airliner last July 3, killing 290 passengers and crew members.

For forces in the Persian Gulf, the rules were tightened after the Stark incident to give commanders in the field "more freedom to exercise command authority" and decide when to fire, said Lt. Jim Wood, a Navy spokesman.

But the ships and planes accompanying the Kennedy were operating under more or less the same rules as they would have been before the Stark or Vincennes incident, said Wood.

"There is a basic premise that the officer in charge must defend his ship at all cost," Wood said.

"In wartime, that means that anything not identified as a friend is an enemy. In peacetime, anything that is not identified as an enemy is a friend," he said. "The peacetime rules of engagement enable our forces to adequately defend themselves when there is danger of hostility."

The judgment of when to fire, he said, is left "to the judgment of the on-scene commanders," in this case the pilot of the lead F-14 on the patrol.

"If you are a pilot leading a flight of aircraft and felt there was imminent danger of being attacked, you don't need to wait to be fired on," said Wood.

Although the decision to fire rests on the unit commander, the rules themselves "are approved at the highest level," said Wood.

"The president ultimately approves them, as he made the change for the Persian Gulf after the Stark incident. They are usually made at the recommendation of the fleet commanders," he said.

Additionally, if unit commanders perceive themselves under greater danger, they may order their weapons kept at the ready without a change in the rules of engagement, he said.

Navy "officers are briefed thoroughly prior to any deployment overseas," he said. Additionally descriptions of the situation in specific regions "are frequently written in classified publications that all officers have access to."

Those guidelines "are dynamic and vary depending on geography. The rules governing ships off the coast of Libya are a lot tighter, for example, than those off the coast of Spain, where there is less perceived danger," he said.