Reuters

The second production model F-35A Lightning II aircraft flies above the compass rose of Rogers Dry Lakebed at Edwards Air Force Base, Calif., in this image distributed by the U.S. Air Force dated May 13, 2011.

The Pentagon said on Thursday it would resume flights of its F-35 fighter jets, which were grounded a week ago after a crack was discovered in an engine blade during a routine inspection.

The cautionary flight suspension began on Feb. 21 after a 0.6 inch crack was found on a turbine blade of a test aircraft at the Edwards Air Force Base F-35 Integrated Test Facility.

Comprehensive tests on the blade were later conducted at the Pratt & Whitney facility in Middletown, Conn., where prolonged exposure to high levels of heat and other operational stressors were determined to be the cause of the crack, a statement from the F-35 Joint Program Office read.

No additional cracks were found during inspections of the remaining inventory, the statement added.

It was the second time F-35s have been grounded in recent weeks. The Pentagon grounded its 25 F-35B jets, used by the Marine corps, on Jan. 18 after a fuel line detached during a training flight.

The F-35B — one of three varieties used by the U.S. — was cleared to resume testing two weeks ago.

The Defense Department touts the Joint Strike Fighter — its most expensive military hardware program, at roughly $400 billion — its "next-generation strike aircraft weapon systems," offering "cutting-edge technologies to the battlespace of the future."

Military planners envision the F-35 as taking a lead role in so-called "first day of war" operations, eventually replacing a range of workhorse jets, including the F-16, the A-10 and the F/A-18. Current plans call for the U.S. to buy 2,443 aircraft under a contract with Lockheed Martin.

NBC News' Courtney Kube and M. Alex Johnson contributed to this report.

Related:

Pentagon grounds all F-35 strike fighters over crack in engine blade

Courtesy U.S. Air Force

An F-22 Raptor fighter jet flies in a training mission over the Nevada Test and Training Range.

Defense Secretary Leon Panetta on Tuesday lifted flight restrictions for a squadron of F-22 fighter jets that are being deployed to Kadena, Japan, within the next few days.

The F-22s were restricted from flying more than a half hour from any landing area and at higher altitudes after several pilots suffered in-flight symptoms of hypoxia. 

The 18 Kadena-bound fighter jets will fly a northern route over the Pacific that will not take them more than 90 minutes from the nearest landing field. Additionally, they will fly at lower altitudes — nowhere near their 50,000-foot operational capacity. The lower altitude will not put pilots under the same physiological stress as higher altitudes.

Once at Kadena, the stealth aircraft will be put back into the current flight restrictions imposed against all other F-22s. Once again on a short tether, pilots will not be permitted to fly more than 30 minutes from a landing field, and must remain at lower altitudes.

Related: Air Force eyes pressure vests in F-22 oxygen deprivation problem

Flight restrictions will remain in place for all F-22s conducting training missions in the U.S. and those deployed to Qatar and the United Arab Emirates in the Persian Gulf region.

The restrictions will be lifted only when the Pentagon is satisfied the Air Force has positively identified the cause and eliminates the threat of hypoxia for F-22 pilots.

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