WASHINGTON - United Technologies Corp expects to pare costs as much as 30 percent over time on the engine it is building for the world's biggest fighter aircraft program, the board chairman told Reuters Wednesday.

The cost of the F135 engine being built by the company's Pratt & Whitney unit is at the heart of a battle with rival engine team General Electric Co and Rolls Royce.

At stake is a potential $100 billion market to power Lockheed Martin's F-35 Joint Strike Fighter, the Pentagon's costliest arms-buying program. The Obama administration is seeking to terminate the GE-Rolls Royce competitor engine on the ground that it is superfluous and would not yield any cost savings.

There will be a time when 30 percent happens, George David, UTX's board chairman, said in an interview after speaking about energy efficiency at the Peterson Institute for International Economics.

He said such economies were typical from the learning curve on an aerospace program when you build volume.

You get numbers like that, he said. Pratt is to deliver its first seven production engines this year for the F-35, which is in early stages of production.

The Pentagon has pressed Pratt to wring out cost savings of about 30 percent, comparable to what it achieved over time on the F119 engine that powers Lockheed's F-22 fighter, Air Force Major General C.D. Moore, the F-35 program's deputy executive officer, told Reuters last week. (Reporting by Jim Wolf; Editing by Gary Hill)