The US military said Tuesday it will not send F-35 fighter jets to Britain's Farnborough air show because of safety precautions, in another embarrassing setback for the most expensive program in Pentagon history.

US officials saw the prestigious show as a chance to stage the plane's international debut in front of potential foreign customers, but instead the cancelled appearance raised yet more questions about the troubled project, which is already years behind schedule.

"While we're disappointed that we're not going to be able to participate in the air show, we remain fully committed to the program itself and look forward to future opportunities to showcase its capabilities to allies and partners," spokesman Rear Admiral John Kirby told a news conference.

The Joint Strike Fighter's appearance had been in doubt after the fleet was grounded following an engine fire last month.

US Navy and Air Force military authorities cleared the planes for flights earlier Tuesday but they imposed several restrictions, including mandatory engine inspections after every three hours in the air, Kirby said.

Given the timing of the show -- which started on Monday and runs through Sunday -- and the restrictions, "this was the most prudent and safe decision," he said.

The required engine inspections represented "a pretty significant limitation in terms of being able to fly them across the Atlantic."

US defense officials took the decision in consultation with their British counterparts, he said.

With the cost rising to nearly $400 billion, the program carries a higher price tag than any previous weapons project.

The Joint Strike Fighter has suffered one technical setback after another and the latest problem has turned into a public relations headache, undermining a long-planned coming-out-party at Farnborough.

US officials chose the Farnborough air show as a way of showcasing the plane in a country that committed to the project early and has invested heavily in the fighter.

- Mishap or deeper problem? -

Apart from the United States and Britain, seven countries are taking part in the program: Australia, Canada, Denmark, Italy, the Netherlands, Norway and Turkey.

Pentagon officials downplayed the engine fire as a one-off "mishap" that came at an inconvenient time.

But it remains unclear how much time will be lost due to the incident and whether it signifies a deeper problem with the engine, manufactured by US-based Pratt & Whitney.

On June 23, an engine caught fire on a F-35 as it was about to take off from Eglin Air Force Base in Florida. The pilot managed to escape unharmed and investigators are still trying to get to the bottom of the incident.

The Pentagon's top weapons buyer, Frank Kendall, has said that an inspection of the whole fleet of aircraft indicates the fire likely was an isolated incident and not part of a broader, systemic problem.

Critics of the plane have seized on the latest problem as proof that the project is deeply flawed.

One US lawmaker, Jim Moran of Virginia, demanded a briefing on the probe into the fire and said the incident "should raise serious concerns about the viability of the program."

Pentagon officials acknowledge that a decision at the outset to start building the jet before testing was finished has caused difficulties. As a result, bugs and other technical glitches have forced repeated repairs and redesign work, slowing down production and raising costs.

Officials and industry executives insist the plane promises to become the ultimate stealth fighter jet, able to evade enemy radar while flying at supersonic speeds. But the plane so far has yet to achieve the level of performance and reliability expected.

Aviation analysts disagree about the prospects for the plane, with some saying the technical problems are not much different from those encountered by other fighter aircraft in early stages of development.

Before Tuesday's decision, the Pentagon had initially planned to send four F-35B aircraft -- the short take-off, vertical landing version -- to the air show, including one plane that is owned by Britain.