FRESNO, Calif. - The grounding of hundreds of F-15s because of dangerous structural defects is straining the nation's air-defense network, forcing some states to rely on their neighbors' fighter jets for protection, and Alaska to depend on the Canadian military.

The F-15 is the sole fighter at many of the 16 or so "alert" sites around the country, where planes and pilots stand ready to take off at a moment's notice to intercept hijacked airliners, Cessnas that wander into protected airspace, and other threats.

The Air Force grounded about 450 F-15s after one of the fighters began to break apart in the air and crashed Nov. 2 in Missouri. An Air Force investigation found "possible fleet-wide airworthiness problems" because of defects in the metal rails that hold the fuselage together. It is not clear when the F-15s will be allowed to fly again.

Compounding the problem created by the grounding, another fighter jet used for homeland defense, the F-16, is in high demand for Iraq operations. And the next-generation fighter, the F-22 Raptor, is only slowly replacing the aging F-15.

Military officials say they moved quickly to patch any holes in the homeland air-defense system, and they report an increase in air-defense sorties in the last month using replacement F-16s. But they acknowledge difficulties.

"When you're filling in, obviously it's going to cause some strain," said Mike Strickler, a spokesman with North American Aerospace Defense Command, or NORAD, which is operated by the United States and Canada. "You're spreading resources a little thinner than we would like."

Air defenses have not been compromised, Strickler said. "We can be anywhere at any time," he said.

With the F-15s in Massachusetts out of commission, the Vermont Air National Guard is covering the Northeast. The Minnesota Air National Guard is manning sites in Hawaii, and Louisiana has gotten help from the Illinois Air National Guard.

And with Oregon's fighters grounded, the California Air National Guard is standing watch for the West Coast. The California Air National Guard said this was the first time that a single state's fighter wing was giving coverage for an entire coast.

The California Guard is keeping three alert sites - in Riverside and Fresno, Calif., and Portland, Ore. - equipped and staffed with pilots and mechanics.

"As a unit we're kind of stressed, but everyone's accepting this as a challenge, and all the men and women of the unit are acting as professionally as you could ever hope for," said Col. Gary Taylor, operations group commander for the Fresno-based 144th Fighter Wing of the California Air National Guard.

The unit has had to borrow F-16s from bases in Indiana and Arizona.

For three weeks in November, Canadian CF-18s filled in for the F-15s over Alaska. Several times, the Canadian fighters scrambled to "do an identification" of Russian bombers flying exercises outside U.S. airspace near Alaska, said Maj. Mike Lagace, a Canadian military spokesman for NORAD.