The United States are counting on additional foreign sales of Lockheed Martin’s F-35 fighter jet to help drive down the plane’s costs.

Air Force Lieutenant General Christopher Bogdan, the Pentagon’s F-35 program chief, told the Senate on Wednesday that he is “cautiously optimistic” about South Korea joining the multinational development program in June when it is due to announce the winner in a procurement competition for the acquisition of sixty planes. Boeing’s F-15 and the Eurofighter Typhoon are among the other systems competing for the contract.

Bogdan also said that Singapore had shown “tremendous interest” in the F-35. The city state, which has a small but highly sophisticated air force, is reportedly in the “final stages” of deciding which plane should replace its three dozen Northrop F-5s which entered service there in the 1970s.

The United States’ Asian allies’ interest in Lockheed’s plane has increased since China unveiled two separate fifth-generation fighter jets of its own which are suspected to have a similar stealth capability.

The American airplane manufacturer and Defense Department claim that the F-35’s capabilities are still superior to those of China’s warplanes.

Even if Singapore and South Korea join the Joint Strike Fighter program, it might not make up for other participating nations reducing their F-35 buys.

Britain, which was supposed to cover some 10 percent of development costs, or $2.5 billion, plans to buy fewer planes as do Italy and the Netherlands which allocated $1 billion and $800 million to the project, respectively. Canada and Japan are increasingly wary as well.

Cost estimates have skyrocketed and varied since Lockheed was awarded the Joint Strike Fighter contract in 2001. The United States Air Force put the F35’s flyaway cost at anywhere between $89 and $200 million in 2010. In February 2011, the Defense Department said the 32 planes it expected to buy the next year would cost almost $210 million each, discounting research and development costs.

The Pentagon expects to finalize the acquisition of the sixth and seventh batch of F-35s by the end of next month, Bogdan told senators on Wednesday. The deal could involve 71 planes at a cost of some $9 billion.

The United States plan to eventually buy several thousands of F-35s which should bring down the costs per individual plane. But the new fighter will also likely be far more expensive in maintenance than the ones it is supposed to replace. The entire program has exceeded its original cost estimate by more than 50 percent. The Americans could pay over $1 trillion to operate the stealth warplanes over the next half century.

As a result of delays in the Lockheed program, the Air Force is refurbishing Boeing F-16 jets, some three hundred of which could remain in service through the 2020s when they would average forty years in service. It will also continue to fly some five hundred F-15s beyond 2030 when the youngest models will be almost fifty years old.

Of the Air Force’s 2,000 fighter jets, less than two hundred F-22 Raptors are currently on the cutting edge of aviation technology.