The US military says it has grounded its F-35 Joint Strike Fighter fleet following a fire on board one of the multi-million-dollar jets.

Directives ordering the suspension of all flights were issued after the fire at Eglin air force base in Florida.

The Pentagon said officials had not been able to pinpoint the cause of the fire, which occurred as a pilot was preparing for takeoff. The pilot was not injured.

Australia announced in April that it would buy 58 of the fighters, in addition to 14 ordered in 2009, at an extra cost of more than $12 billion.

The Pentagon said preparations for F-35s to participate in two air shows in the United Kingdom later this month would continue, with a final decision to be made early next week.

At $US398 billion, the F-35 program has become the Pentagon's costliest.

The fire was the latest incident to hit the program and followed an in-flight oil leak that triggered mandatory fleet-wide inspections of the jets last month.

"Additional inspections of F-35 engines have been ordered ... return to flight will be determined based on inspection results and analysis of engineering data," the US Defence Department said in a brief statement.

The Pentagon's F-35 program office said determining the root cause of the fire and potential mitigating actions were its highest priority.

It said impacts to flight test, training and operations of the radar-evading warplane were being assessed.

Reuters/ABC