Production of early models of the F-35 Lightning II should slow down to help reduce costs after potential cracks and "hot spots" were found following testing, the U.S. military's program head says.

"The analyzed hot spots that have arisen in the last 12 months or so in the program have surprised us at the amount of change and at the cost," U.S. Vice-Admiral David Venlet said in an article posted online Thursday by AOL Defense.

"Most of them are little ones. But when you bundle them all up and package them, and look at where they are in the airplane and how hard they are to get at after you buy the jet, the cost burden of that is what sucks the wind out of your lungs," he said.

Venlet said the discovery of weaknesses in the fighter is not surprising.

"It's a fighter made out of metal and composites. You always find some hot spots and cracks and you have to go make fixes. That's normal," he said.

The aircraft, also known as the Joint Strike Fighter, is still in early production and testing. AOL Defense says only 18 per cent of flight testing has been completed and fatigue testing is in initial phases.

The U.S. government has been buying small quantities of the fighter, and paying for design changes and required retrofits. A fifth batch of fighters — dubbed the Low Rate Initial Production Lot 5 contract — is under negotiation. The U.S. government is pushing Lockheed Martin, the prime contractor, to pick up part of the cost for changes in design and parts.

Slowing production now would help reduce costs of replacing parts on jets completed before testing is finalized, Venlet said.

He also indicated the future of the jet is not in question, stating that the changes are not required for safety but to lengthen the jet's working life.

"The question for me is not: 'F-35 or not?'" Venlet said. "The question is, how many and how fast? I'm not questioning the ultimate inventory numbers, I'm questioning the pace that we ramp up production for us and the partners, and can we afford it?"

The United States plan to buy more than 2,400 F-35 fighters at a cost of more than $380 billion US is expected to be the largest arms purchase in history.

Canada plans to buy 65 of the aircraft.

Prime Minister Stephen Harper said Friday that the government has no intention of changing its plan.

"I'm not going to comment on every single rumour," he said at an event. "All the information we have is that we remain on track and that we're at this point going to continue obviously working with the industry and our allies on the development of this aircraft."

Harper added that more than 60 Canadian companies already have contracts associated with the aircraft.

Earlier this month, Canadian Defence Minister Peter MacKay and U.S. Secretary of Defence Leon Panetta dismissed speculation that budget pressures will cause their countries to pull back from their F-35 jet purchases.

The Canadian Press also reported this month that delivery of the jets will spread out between 2016 and 2023, with most arriving after 2019, according to internal Defence Department figures. Canada's current fleet of 77 CF-18s is due to be retired by 2020.