US military officers are still unsure if the F-35 fighter will be cleared to make its debut at a British air show next week, a potentially damaging setback for the costly program.

The Joint Strike Fighter has been touted as a technical marvel but the fleet remains grounded as authorities investigate what caused an engine fire last month for one of the aircraft, the Pentagon said on Tuesday.

Officials had viewed the July 14-20 Farnborough International Air Show as a promising opportunity to show off the new plane before a global audience in Britain -- a country that has invested heavily in the project.

But the coming-out party may be called off over safety concerns, officials said.

"There has not been a decision made on that yet," Pentagon spokesman Rear Admiral John Kirby told reporters when asked whether the plane would fly at the event as planned.

"I think certainly we'd be disappointed if we weren't able to take it to Farnborough.

"That said, safety's got to be priority number one, and it is. And nobody wants to rush these aircraft back into the air before we know exactly what happened."

Four F-35Bs, the vertical take-off version designed for the US Marine Corps, were supposed to feature in the air show, including one aircraft belonging to Britain that is undergoing tests in the United States.

Both the US Air Force and Navy last week ordered a halt to all F-35 flights following the June 23 engine fire to allow for inspections of all the aircraft.

The blaze broke out at the rear of the plane on the runway during takeoff at Eglin Air Force Base in Florida, according to the Air Force. The pilot managed to exit the jet safely and the fire was put out, but officials have yet to say what exactly triggered the problem.

- 'Fully committed' -

The incident raised fresh questions about the F-35 program and whether it can deliver as a game-changing stealth fighter. The most expensive weapons project in US history has been plagued by repeated delays and cost overruns.

The program's costs have swelled to nearly $400 billion for more than 2,000 aircraft, with each plane costing $160 million.

The engine fire came just as the program appeared to be gaining a more solid footing, with officials pointing to progress in keeping costs under control and staying on schedule.

Kirby said military leaders had full confidence in the aircraft and that Defense Secretary Chuck Hagel was due to pay a visit on Thursday to the Eglin base where the engine fire took place.

"The secretary's visit particularly at this time sends a strong message to our international partners that the United States remains fully committed to the F-35 program," he said.

Defense officials told AFP it was possible that three of the four planes could take part at Farnborough toward the end of the show, if aviation authorities conclude there is no serious risk to safety.

"There's still some time to make a decision," said an official who spoke on condition of anonymity.

Officials had originally planned for the F-35B aircraft to participate this weekend in a military aviation display, the Royal International Air Tattoo, but it was unclear if authorities would be ready to take a decision in time.

British military aviation rules are stricter than US regulations and it is looking increasingly unlikely that the British-owned F-35B -- now at Eglin in Florida -- would make the trip across the Atlantic Ocean, officials said.