The US Air Force announced Tuesday a two-year delay in the production of its new F-35 stealth fighter, touted as the future of the American air fleet.

Secretary of the Air Force Michael Donley, the civilian head of the department, told reporters that the Joint Strike Fighter, scheduled for use by 2013, would now not be ready until the end of 2015.

"I do think we're going to have a slip" in the schedule, Donley said, raising fresh questions about the F-35 project, involving nine other countries and portrayed as a model for a more efficient, streamlined weapons program.

Defense Secretary Robert Gates a month ago sacked the general in charge of the F-35 program and said he would withhold 614 million dollars in performance fees from contractor Lockheed Martin over a series of cost overruns and delays.

Donley said the Pentagon was pushing Lockheed to speed up its work and keep its costs under control.

"We want to hold the contractors' feet to the fire," he said. "We want to incentivize them to make good on the promises they made earlier and deliver on schedule."

Much is riding on the stealth aircraft, which Gates has held up as the premier US fighter jet after pushing through an end to the costly F-22 Raptor project despite opposition from some lawmakers.

The Defense Department plans to buy nearly 2,500 of the jets over the next 25 years, at an estimated cost of about 300 billion dollars.