US military officials have approved limited flights for Lockheed Martin's F-35 fighter (Joint Strike Fighter), but the next-generation combat jet will not make its anticipated international debut before potential buyers this week.

The F-35, the world's most expensive weapons project with a price tag of about $400 billion, has been grounded for almost two weeks since the massive failure of a Pratt & Whitney engine on a US Air Force F-35 at a Florida air base last month.

Australia is planning to buy 72 of the new F-35 fighters at a cost of more than $12 billion.

Pentagon spokesman Rear Admiral John Kirby said US Air Force and Navy officials had granted the radar-evading jet a limited flight clearance.

The clearances are conditional on engine inspections and speed limitations, which will prevent the jet from breaking the sound barrier.

The front fan section of the fighter jet's engines must be inspected after three hours of flight time.

A ceremony to celebrate the first jet rollout is due to go ahead next week.

But Rear Admiral Kirby has said the jet will not make a much anticipated debut at the Farnborough air show in southern England this week.

"I can confirm that the Department of Defense in concert with our partners in the UK has decided not to send Marine Corps and UK F-35B aircraft across the Atlantic to participate in the Farnborough air show," Rear Admiral Kirby said.

"We're actually glad for the news today to get the aircraft back in the air even if it is limited - we fully expect to work our way through this problem and restore the aircraft to full operational capability in the near future.

"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."

The jet's failure to appear at a big military air show in Britain last week, and its absence from Farnborough, is a blow for US officials and their international partners, who were hoping to showcase the capabilities of the new multi-role fighter.

More than 3,000 orders for aircraft so far

Global orders for the F-35 are expected to exceed 3,000, with Italy, Turkey and Canada also among US allies planning to purchase the plane.

"It's a black eye and a PR nightmare, but it's not going to change the outcome," said aerospace analyst Richard Aboulafia, from the Virginia-based Teal Group.

He said the jet's absence from the British show was unlikely to affect buying decisions by foreign military forces, but could prompt Canada and Denmark to opt for a competition instead of an outright F-35 purchase.

The reaction from Lockheed Martin was muted.

"While we were looking forward to the F-35 demonstration at Farnborough, we understand and support the DoD and UK Ministry of Defence's decision," spokeswoman Laura Siebert said.

Rear Admiral Kirby said as well as the requirement for inspection of the fighter jet's engines, restrictions on the plane's return to flight also included limiting its speed to 0.9 Mach and 18 degrees of angle of attack.

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

The decision is sure to disappoint top executives from the biggest contractors involved in the F-35 program, who had travelled to Britain for the plane's foreign debut.

F-35's grand debut plugged for months

Billboards all over London have been heralding the F-35's grand debut for months.

The planes had been slated to follow a route relatively close to the US and Canadian coast, up past Greenland before heading to Europe, rather than a direct flight across the Atlantic Ocean, according to sources familiar with the plans.

US authorities say they still have not determined the cause of a fire on an F-35 last month which lead to the grounding.

The decision to lift the grounding order was made at a high-level meeting on Monday and reflects growing evidence the engine failure was a one-off event and not due to a systemic or fundamental design flaw, sources familiar with the matter said.

Pentagon acquisition chief Frank Kendall told reporters no similar problems had been found on any of the other 98 engines in service and underscored that the program was still in the development stage, when technical problems were meant to be found and fixed.

Pratt president Paul Adams said the engine failure and a separate incident in May involving its CSeries commercial engine were unrelated and did not point to a larger problem.

Mr Adams, who took over as president in January, said it had been a "challenging few weeks", but that both engines were still going through the developmental stage aimed at flushing out problems and resolving them.

Reuters/ABC