WASHINGTON - The military has flown more than 13,000 fighter-jet patrols over American cities since Sept. 11 at a cost exceeding $324 million. Now it wants to cut back.

The round-the-clock patrols designed to deter terrorists may be straining planes and personnel, the Pentagon said yesterday.

Four months after the airliner attacks, any decision on ending or changing the patrols may come down to a calculation of how safe Americans would feel with the change, some officials say.

Part of the homeland defense efforts called Operation Noble Eagle, the flights began after terrorist hijackers crashed jetliners into the Pentagon and World Trade Center. U.S. fighters have been flying over New York and Washington since then.

Other patrols fly from time to time over other major metropolitan areas and key sites, and jets are on alert at 30 bases to scramble if called. The combat air patrols are the first of their kind over the United States since the Cuban missile crisis in 1962.

Officials have been looking to cut back on the program for some time, knowing from the outset that the high-tempo use of manpower and equipment couldn't keep up with the existing people and budget, one defense official said, commenting only on condition of anonymity. Now that four months have passed and aviation security has been improved somewhat, some wonder if it might be time to start rethinking the patrols, the official said.

Pentagon spokeswoman Victoria Clarke declined to confirm that yesterday, telling a Pentagon press conference that talking about details of the program could give "an advantage to those who might want to do us harm."

The operation uses 11,000 people and 250 aircrafts, another official said. Those figures include maintenance crews, pilots for 100 F-15 and F-16 fighter jets, crews for tankers needed for midair refueling and crews for AWACS - Airborne Warning and Control System - planes to provide radar information.

Rear Adm. John Stufflebeem said that AWACS pilots and crews may be operating so intensely that they are not getting usual training for other missions.

"Maybe we're not getting the training that we need done now for our rotations overseas, so that's being looked at," he said at the press conference with Clarke. Stufflebeem is deputy director of operations for the Joint Chiefs of Staff.

The fighter pilots, mostly from Air National Guard units, do patrols of two to six hours. The jets are refueled about every two hours, meaning some go through two midair refuelings.

From Sept. 11 to Dec. 10, the operation flew 13,000 sorties. The cost was $324 million, Defense Department spokeswoman Susan Hansen said.

The North American Aerospace Defense Command, which runs the operation, said periodic review of missions is standard military procedure.

"We continuously analyze our ongoing operations ... as a matter of prudent military planning," said Maj. Barry Venable, spokesman for NORAD in Colorado Springs, Colo.

NORAD says that through Dec. 10, its jets responded 207 times to problems such as unidentified aircraft, planes violating restricted air space and in-flight emergencies.

Not included in the figure is the case in which two jets escorted a Paris-to-Miami flight to Boston later last month after a passenger tried to ignite what authorities said was an explosive hidden in his shoes.

In 92 of the cases, jets on alert on the ground scrambled to respond.

In the other 115 cases, NORAD diverted jets that already were in the air on patrol.

Pentagon officials said privately that the longer the program continues at its current rate, the higher the stress that could eventually harm readiness for other missions.

While they believe patrols deter would-be attackers and give Americans a greater sense of security, they also argue that scrambling planes against attacks is a measure of last resort. Security should be tightened on the ground before problems become airborne in the first place, they maintain.

One alternative to constant patrols would be to keep planes on ground alert.

In addition, airliner and airport security has been tightened in the past few months. Thousands of National Guard troops are on duty at the nation's airports. Screening of passengers and carry-on baggage has been increased.

Under a new aviation security law, airlines are required starting next weekend to inspect all checked baggage for explosives.