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The U.S. military said it had grounded the entire fleet of 97 Lockheed Martin Corp F-35 fighter jets until completion of additional inspections of the warplane's single engine built by Pratt & Whitney, a unit of United Technologies Corp.

The Pentagon's F-35 program office, Air Force and Navy issued directives on Thursday ordering the suspension of all F-35 flights after a June 23 fire on an Air Force F-35A jet at Eglin Air Force Base in Florida, according to statements by the Pentagon and the F-35 program office.

The Pentagon said U.S. and industry officials had not pinpointed the cause of the fire, which occurred as a pilot was preparing for takeoff. The pilot was not injured.

The Pentagon said preparations were continuing for F-35 jets to participate in two UK air shows later this month, but a final decision would be made early next week. The fire has already derailed plans for an F-35 jet to fly by a naming ceremony for Britain's new aircraft carrier on Friday.

Pentagon's costliest weapons program

The June 23 incident was the latest to hit the Pentagon's costliest weapons program, the $398.6 billion F-35 Joint Strike Fighter. It followed an in-flight oil leak that triggered mandatory fleetwide inspections of the jets last month.

'Additional inspections of F-35 engines have been ordered, and return to flight will be determined based on inspection results and analysis of engineering data,' the Defense Department said in a brief statement issued late on Thursday.

Pratt & Whitney said it was working closely with Air Force officials who are investigating the fire to determine the cause of the incident and inspect all engines in the fleet. Spokesman Jay DeFrank said it would be inappropriate to comment further since the incident was the subject of an investigation.

The Pentagon's F-35 program office said determining the cause of the fire and potential mitigating actions were its highest priority. It said impacts to flight test, training and operations of the radar-evading warplane were being assessed.

Problem not understood, yet

A person familiar with the situation said it was premature to rule in or out any quality problem or manufacturing defect.

Air Force Lieutenant General Chris Bogdan said the F-35 program office would work closely with the military and industry as engineering findings were analyzed, and would participate in decisions about resuming F-35 flights

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'We will contribute to the return to flight determination, and will aim to do what is prudent for the enterprise at large without compromising the ongoing mishap investigation,' he said.

Reuters reported on Wednesday that U.S. and British authorities were preparing directives ordering a mandatory engine inspection estimated to take about 90 minutes.

British officials remained part of the discussions with U.S. officials and concurred with the U.S. recommendation to ground the jets, pending further inspection results, the F-35 program office said.

The Royal International Air Tattoo (RIAT) kicks off July 11 followed by the Farnborough International Air Show starting on July 14.