The Air Force has temporarily grounded its fleet of B-1B bombers in a precautionary move following an incident Monday in which an internal fuel tank on one of the bombers was punctured, the Pentagon announced Tuesday.

The incident occurred at Dyess Air Force Base, Texas, during a routine pre-flight check before a B-1B training flight, said Dan Howard, the Pentagon's chief spokesman. The fuel tank appears to have been punctured when the mechanism that sweeps the bomber's wings back and forth malfunctioned, he said.Howard, disclosing the grounding during a regular Pentagon news briefing, said the bomber's crew was sweeping the wings forward and backward before takeoff and discovered the wings were not moving evenly together. "They were out of sync," Howard said.

"The third time that they swept the wings forward, a clunking noise was heard; they shut down the aircraft, and they observed the fuel leak. They're conducting an investigation to see what caused the problem."

Howard said it appeared the left wing, in being swept to a full forward position, "exceeded its normal limits and subsequently fuel was noticed in the wing root area."

The Strategic Air Command has suspended training flights on its B-1B aircraft until further notice, Howard said. The Air Force has lost three of its 100 B-1B bombers to crashes.

Air Force officials also are conducting an investigation to find out what caused a 12-foot section of a wing flap to fall off a B-52 shortly after takeoff from Fairchild Air Force Base, near Spokane, Wash.

The plane, which was on a routine training flight, made a successful landing Monday after circling to burn off fuel, said Capt. Brad Peck, a base spokesman. He said a section of the plane's right inboard flap fell off, but there were no injuries.