WASHINGTON, March 15 (Xinhuanet) -- The U.S. Defense Department is scheduled to spend 49.3 billion U.S. dollars on 424 F-35 Joint Strike Fighters before the next-generation fighter has been fully flight-tested, the Government Accountability Office (GAO) said Wednesday.

    Production of the jet fighters before the design is mature through flight testing may increase its cost significantly and push the program behind schedule, the GAO, the Congress's investigative agency, said in a report.

    "Producing aircraft before testing demonstrates the design is mature increases the likelihood of design changes that will lead to cost growth, schedule delays and performance problems," it said.

    According to the report, the Pentagon plans to purchase 424 Joint Strike Fighters by 2013, but only 98 percent of flight testing will be completed by then.

    The report urged the Pentagon to adopt more cautious, step-by-step contracts for development work, and delay procurement of the aircraft until it is fully flight-tested.

    The United States, and eight of its allies, including Australia, Britain, Canada, Denmark, Italy, the Netherlands, Norway and Turkey, have taken part in the program, which was designed to produce 3,100 F-35 fighters over 30 years.

    Lockheed Martin Corp. was awarded a contract in 2001 to develop the fighter plane, and the first five aircraft are scheduled to be produced in 2007. Enditem