Four Air Force Thunderbird aerobatic jets failed to pull out of a loop and slammed into the Nevada desert outside Las Vegas yesterday, killing all four pilots in the worst disaster of its kind.

Eyewitnesses said a giant ball of flame shot into the sky after the four planes dove into the ground one after the other only yards apart at 1:43 p.m. EST.

A photographer who flew over the crash site at Indian Springs, about 40 miles northwest of Nellis Air Force Base, said the impact broke the T38 Talon trainer jets into tiny pieces, with the largest no larger than a car fender.

The Air Force said it had not determined the cause of the crash but would spend the night sifting through the debris.

The four planes went into the loop in a row, wingtip to wingtip, and zoomed toward the earth at about 400 miles an hour. They were supposed to pull out of the loop 100 to 200 feet above the ground.

In this "line abreast" loop, three of the four pilots key their moves to those of the plane to the left. The farthest left plane is the leader, meaning the other three are supposed to do exactly what it does.

The crash could have occurred because the command plane's pilot did not pull out in time, bringing the other three down with him, or because there were collisions in mid-air at some point in the maneuver.

An Air Force spokesman at the Nellis base, home of the Thunderbirds, said the wingtips are only about six feet apart when the abreast loop is performed at an air show but are farther apart during practice sessions.

The Thunderbirds were practicing for an air show on March 13 at Davis-Monthan Air Force Base in Tucson.

The four-abreast loop is not considered as dangerous as some other air show maneuvers, such as the "bomb burst," where the planes race toward one another from different directions and then shoot high into the sky.

Yesterday's crashes were the worst ever experienced by Air Force or Navy aerobatic teams. The Air Force said that before yesterday the Thunderbirds had lost 15 air crew members since the air show program started in June, 1953.

The worst previous disaster for military aerobatics was in July, 1973, in Lakehurst, N.J., when two Navy pilots and a crewman were killed practicing for a show.

A major military rationale for conducting such dangerous maneuvers is to kindle enthusiasm for air careers among young people. In the wake of yesterday's crashes, however, Air Force officers were predicting that planned air shows will be called off, at least for the rest of this year.

The Air Force was shocked and saddened by the crashes. Thunderbird pilots are the hotshots of peacetime fliers, the darlings of the towns they perform in, and celebrities around their home base. This made it all the more tragic for those who saw the planes plunge into the desert on the clear day.

"We saw it coming," said Thomas Sullivan of Boulder City, who was working on a construction project nearby. "I watched the planes do a loop and they didn't pull out. The one farther east hit the ground first. The other three followed within a tenth of a second, flying in formation. It was the wrong altitude. They didn't pull up fast enough. I couldn't believe they crashed. It was a ball of flame, just like a napalm bomb."

Jim Kelso of Ojai, Calif., said he was driving when he saw the four planes zooming down toward the desert.

"Just as they pulled out of the dive, all four of them hit the ground," he said. "The instant they hit, you knew they were dead."

Kelso's description made it sound as if the pilots almost made it out of their loop. Air Force spokesmen said the traditional maneuver is to fly 100 to 200 feet above the ground, shoot up to between 2,000 and 3,000 feet and then loop back down to the original path without breaking the wingtip-to-wingtip formation.

The Air Force identified the pilots as Maj. Norman L. Lowry III, 37, of Radford, Va., the team leader; Capt. Willie Mays, 32, of Ripley, Tenn.; Capt. Joseph Peterson, 32, of Tuskegee, Ala., and Capt. Mark E. Melancon, 31, of Dallas.

All of the pilots had been assigned to the aerobatics team for less than two years.

In Air Force shows, four Thunderbirds fly in formation and two others conduct solo stunts. The Thunderbirds have flown in air shows in 50 states and 45 foreign countries and have been watched by 154 million people, according to the Air Force.