But a senior administration official said the option would be chosen only in a narrow circumstance: if Moammar Kadafi ordered large-scale air attacks against fellow Libyans.

“If Kadafi begins an all-out assault on the opposition, creates a humanitarian crisis, and it includes significant use of air power, then that would be the kind of compelling situation that spurs the international community to take that step,” said the official, who spoke on condition of anonymity because of the sensitive nature of the discussions.

If a no-fly zone is implemented, one of the biggest worries for U.S. planners will be Libya’s surface-to-air missile batteries along its coastline, especially its so-called SA-6 missiles, which, though designed years ago by the Soviet Union, remain able to shoot down U.S. and European fighters, several analysts said.

Libya is believed to have about 50 SA-6 missiles, which are easy to move to avoid detection.