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.

Canada is poised to buy 65 Lockheed Martin Corp F-35 Joint Strike Fighter jets, but the $400 billion program's development has been plagued by technical issues, with many critics arguing that more viable alternatives to the jet exist on the market. (U.S. Air Force/Reuters)

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 Pratt & Whitney engine that caught fire in an F-35 in Florida is the same model that powers all three derivatives of the joint strike fighter. (Lockheed Martin/Randy A. Crites/Reuters)

The Pentagon's F-35 program office has made determining the cause of the fire its highest priority and it is assessing the impact on flight tests, training and operations of the radar-evading warplane.

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

Both Australia and South Korea said there had been no change to their plans to buy the fighter jets. Australia plans to buy 58 of the fighters and South Korea intends to buy 40.

Problem not understood, yet

"To date the JSF aircraft has accrued 15,000 flight hours While the F135 engine has successfully completed nearly 32,000 hours of testing," a spokesman for Australia's Defence Minister David Johnston said.

"Single engine fighters are operated by many air forces and Defence remains confident the F-35 JSF will be reliable and safe."

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.