Washington - The head of the US Air Force on Thursday said she would be "concerned" if President Barack Obama were to formally declare a "no-first-use" policy for America's arsenal of nuclear weapons.US media reports have in recent weeks said Obama is weighing an overhaul of long-standing nuclear policy, including by pledging to never conduct the first strike in a nuclear conflict."I would be concerned about such a policy," Air Force Secretary Deborah Lee James told the New America think tank in Washington."Having a certain degree of ambiguity is not necessarily a bad thing. You certainly want to communicate certain things to allies and to your potential adversaries around the world, but you don't necessarily want to show all your cards all the time."The US Air Force is responsible for two legs of America's nuclear defence "triad" - a three-pronged nuclear system comprising long-range bombers, ground-launched missiles and submarines.The Washington Post last month reported that Obama, who has espoused a world free of nuclear weapons, is weighing a range of measures that he could implement before leaving office at the end of the year.Among these measures are funding cuts to modernise America's nuclear arsenal, and cancelling or delaying development of the Long-Range Stand-Off nuclear cruise missile, the Post reported.Separately, James reacted to this week's news that an initial squadron of Air Force F-35 jets is finally ready for combat.At nearly $400bn, the most expensive plane in history has been beset by delays, technical problems and cost overruns."It has taken too long, there were too many schedule slips and of course it's gone over budget, and that's the part [of the plane's history] I would like to re-write," she said.But "the thing I am bullish about is it's exactly the type of aircraft we need for some of these high-end threats around the world that we believe are going to be the key threats of the future".