WASHINGTON- 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.

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 incident is the latest to hit the Pentagon's costliest weapons program, the $398.6 billion F-35 Joint Strike Fighter.

"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.

Pratt & Whitney said it was working closely with Air Force officials who are investigating the fire and are inspecting all engines in the fleet. 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.