The engine problem was discovered during a routine inspection at Edwards Air Force Base, California, leading the 51-jet fleet to be grounded as a "precautionary matter," the Pentagon said Friday.

"It's too early to know the fleet-wide impact of this finding, however as a precautionary measure, all F-35 flight operations have been suspended until the investigation is complete and the cause of the blade crack is fully understood," said program spokeswoman Kyra Hawn in a statement.

The cracked turbine blade was being shipped to manufacturer Pratt and Whitney's engine plant in Middletown, Connecticut for an evaluation. None of the grounded planes have been fielded for combat operations and all are undergoing testing.

At $400 billion (300 billion euros), the F-35 is the Defense Department's most expensive weapons program. The military had planned to produce 2,443 planes, which are supposed to replace most of the combat aircraft fleet in the Air Force, Navy and Marine Corps, but some members of Congress have balked at the cost.

The fleet will remain grounded until the cause of the problem has been determined.

dr/jr (AP, AFP, dpa, Reuters)