The Pentagon is resuming test flights of its new-generation F-35 fighter while investigating the cause of an engine fire last month, but the plane will operate with restrictions and will not fly in the Farnborough International Airshow in England.

The Pentagon's press secretary, Navy Rear Adm. John Kirby, said it was a "difficult decision" to cancel the hoped-for participation at Farnborough, but the Pentagon remains confident that any of the plane's technical issues can be fixed.

"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 to partners," Kirby said.

Kirby said that while limited flying has been approved for the aircraft, there are a number of restrictions that would make it difficult for the fighter jets to cross the Atlantic to the airshow. For example, after a maximum of three hours of flight the front fan section of the engine must be inspected, he said.

"That was a pretty significant limitation in terms of being able to fly them across the Atlantic," he added.

Farnborough would have been the F-35's first international air show.

The entire fleet of nearly 100 planes was grounded after a fire at Eglin Air Force Base in Florida on June 23. Kirby said inspections so far have not revealed a systemic problem and defense officials feel "increasingly comfortable" that the aircraft will be able to return to full flight at some point.

The F-35 is the Pentagon's most expensive aircraft program, costing an estimated $400 billion.

Three variants of the aircraft are being developed and built, one each for the Air Force, Navy and Marine Corps.

The Marines expect to field their version next summer, the Air Force will follow in 2016, and the Navy a year after that. International buyers include Britain, South Korea, Israel, Italy, Australia, Canada, Turkey and Japan.

Members of Congress, however, have complained that the program has been troubled with testing problems, delays and cost overruns. The jet is intended to replace Cold War-era aircraft such as the Air Force F-16 fighter, the Navy's F/A-18 Hornet and the Marines' EA-6B Prowler and AV-8B Harrier.