Australia's biggest Defence acquisition is currently rated as "unaffordable" because of reliability issues, a US congressional committee has heard.

The F-35 Joint Strike Fighter (JSF) has been billed as the smartest fighter jet on the planet, designed to strike enemies in the air and on the ground without being detected by radar.

The first two of Australia's initial order of 14 F-35s are expected to be delivered this year at a cost of just under $US130 million each.

Federal Cabinet's national security committee is expected to endorse the purchase of an additional 58 F-35s next month.

But the head of the JSF program, US Air Force Lieutenant General Chris Bogdan, visited Australia two weeks ago and declared the reliability and maintainability of the aircraft was not yet "good enough".

And overnight the US House Armed Services Committee was told the planes are currently not affordable to use at the moment.

The committee heard software problems could delay the fighter's production, and foreign buyer delays could see countries like Australia paying millions of dollars more per aircraft.

"We've looked at the reliability too and it is a really big concern now - it's very risky," the General Accounting Office's Michael Sullivan said.

"Not just in terms of getting the unit costs down on the aircraft, but also in terms of the operating and support costs.

"The estimate now is deemed unaffordable.

"That's all got as much to do with reliability of the aircraft as anything else.

"This is a critical point."

California Democrat Loretta Sanchez told the hearing that all three versions of the aircraft were below their planned reliability.

"If you're paying for it but not flying it, that's bad news," she said.

Parts coming off the aircraft 'too frequently': Bogdan

Ms Sanchez says the F-35 is currently four hours between "critical failures" rather than 13 hours as expected, and at that rate the program will not even meet its reliability goal of 50 per cent.

Lieutenant General Bogdan said with more planes in the skies, program bosses now know parts are coming off the aircraft "too frequently" for maintenance.

"The problem here is you're not going to see results in the next two to three months," he said.

"It's going to take months and months and months of constant efforts to see this improve.

Sorry, this video has expired F-35 threatens higher costs for Australia

"Our goal is by 2015 to see the aircraft at 60 per cent (reliability)."

Ms Sanchez said the reliability target is a "critical price to this program".

"You and I need to keep an eye out and ensure this reliability continues to go up, rather than stagnate as it is," she told the hearing.

The committee heard countries like Australia may risk paying millions of dollars more per aircraft because Italy, Turkey and Canada have or are considering delaying their purchases.

"If those three partners choose to push airplanes out or reduce their buy, it will have an effect on all the other partners and the services buying the aircraft to the tune of about 2-3 per cent increase in price," Lieutenant General Bogdan said.

He and Mr Sullivan agreed that the program's biggest risk is software development.

"The longer it takes to complete that software development, of course the longer you remain concurrent between testing and production, and that means more changes could take place before you finally get the aircraft that you want," he said.

"All that stuff creates costs and inefficiency."

The F-35 is already in production but is still being designed and refined.

The hearing was told the US Air Force has 58 operational F-35s, which have flown 12,000 hours.

Manufacturers Lockheed Martin are expected to build and deliver 35 more aircraft this year.