All steps to improve the reliability of the US-made F-35 aircraft have "stagnated" due to numerous technical issues, Bloomberg wrote, referring to a report by the Pentagon.
Only some 50% of the fighter jets are ready to be used in the battlefield, and this figure has not significantly improved since October 2014, despite the fact that the number of F-35 aircraft produced has increased.
New versions of the plane's complex software have been modified 31 times, however, some key shortcomings still remain in place, the report says, adding that they can seriously impact the effectiveness of the aircraft's combat use.
Moreover, some jets cannot be refueled in the air, while other technical defects concern the launch of AIM-120 air-to-air missiles and the release of air-ground munitions.
READ MORE: Hot Air: Costly US F-35 Fighter Jets Miss the Mark, Denmark Finds
The new tests, that are considered "the most credible means to predict combat performance," are likely to be delayed for a year and won't be completed until December 2019, according to the testing office.
Back in November, Danish authorities already voiced their criticism of the new F-35 fighter jets, which they had purchased from the US. They, in particular, stated that the aircraft yields a much more modest performance than it was initially expected.