The Pentagon has proposed a plan to reduce the number and frequency of round-the-clock combat air patrols flown over American cities since the Sept. 11 terrorist attacks, a spokeswoman said Monday. 

"We are looking at a plan ... that will employ some kind of mix of combat air patrols over certain locations," Pentagon spokeswoman Victoria Clarke said. 

Clarke said the plan also would use "intermittent combat air patrols on an ad hoc basis," and put fighter jets at various military bases on "strip alerts," which puts them on 15-minute notice for combat duty. 

"It will be a very fluid mix that we can and will adjust as the threat conditions demand," Clarke told reporters. 

The spokeswoman said she did not know of a definite date for implementing the plan. 

President Bush has been fully briefed on the matter, she said. 

In January, the Air Force's top civilian official said the patrols — composed of F-15s and F-16s — were putting too much strain on the Air Force. 

"That was never intended to be a permanent thing," said Air Force Secretary James Roche, referring to the idea of having fighters in the air at all times over Washington and New York in case of another suicide hijacking. 

In testimony Thursday before the House Armed Services Committee, Gen. Ralph E. Eberhart said more than 19,000 flights have been conducted in U.S. and Canadian air space in support of Operation Noble Eagle, the military's name for the effort. Eberhart is the Air Force general in charge of the North American Aerospace Defense Command, which is responsible for the overflights. 

Besides covering New York and Washington, other flights took place from time to time over other major metropolitan airports and critical security sites. 

The anti-terrorism patrols required the Air Force to devote 260 aircraft — including fighters, AWACS radar control planes and C-130 transport planes — and 350 air crews to the mission. 

Pentagon officials had said that besides the wear and tear on the aircraft, the daily missions were harming opportunities for pilots and crews to train for their normal overseas missions. 

Clarke said the proposed change in patrols was made possible by the increased security at civilian airports and for commercial airliners. 

The fighters were authorized to shoot down a hijacked airliner, senior Bush administration officials have said. 

The post-Sept. 11 combat air patrols were the first of their kind over the United States since the Cuban missile crisis in 1962.