The shortage of European munitions, along with the limited number of aircraft available, has raised doubts among some officials about whether the United States can continue to avoid returning to the air campaign if Libyan leader Moammar Gaddafi hangs on to power for several more months.

U.S. strike aircraft that participated in the early stage of the operation, before the United States relinquished command to NATO and assumed what President Obama called a “supporting” role, have remained in the theater “on 12-hour standby” with crews “constantly briefed on the current situation,” a NATO official said.

So far, the NATO commander has not requested their deployment. Several U.S. military officials said they anticipated being called back into the fight, although a senior administration official said he expected other countries to announce “in the next few days” that they would contribute aircraft equipped with the laser-guided munitions.