WASHINGTON (Reuters) - The Pentagon and U.S. Navy on Friday ordered a temporary grounding of the Marine Corps version of Lockheed Martin Corp's F-35 fighter jet after an incident that occurred during a training flight at Eglin Air Force Base in Florida on Wednesday.

The Pentagon's F-35 program office said the grounding affected all 25 F-35B model jets, while flights of the Air Force's A-model and the Navy's C-model were unaffected. Ground operations of the B-model planes continued, it said.

The program office it ordered the suspension of flight operations after a propulsion line associated with the B-model's exhaust system failed prior to takeoff. The pilot aborted the takeoff without incident and cleared the runway, the program office said in a statement. There were no injuries to the pilot or ground crew.

The incident came just days after the Pentagon's director of testing and evaluation released an 18-page report detailing an array of problems which he said underscored the "lack of maturity" of the $396 billion fighter program.

Matthew Bates, spokesman for Pratt & Whitney, a unit of United Technologies Corp, which builds the engines for the singe-engine, single-seat fighter jet, said an initial inspection discovered a detached propulsion line in the rear part of the engine compartment.

The fueldraulic line is not used in the A- or C-models, which are still permitted to fly.

"A team of Pratt & Whitney and Rolls-Royce engineers is investigating the cause of the incident and working closely with Lockheed Martin and the F-35 Joint Program Office to resolve the matter," Bates said in a statement provided to Reuters.

He said there had been no previous issues with the component that triggered the grounding.

(Reporting By Andrea Shalal-Esa; Editing by Bernard Orr)