The estimated development costs of the ongoing modernization of F-35 fighter jets have increased by $1.9 billion since 2019, and amid delays, it is not yet certain whether the program will be extended through 2027, according to a new report by the U.S. congressional auditors.

In its Thursday report, the Government Accountability Office (GAO) said the Pentagon and the Lockheed Martin program for "Block 4" upgrade work on the next generation joint strike fighters are behind schedule, while there are some positive signs, such as finding alternative producers of parts for the jets.

The report stressed that the Pentagon has identified alternative suppliers for 1,005 parts produced by Turkish companies as of December 2020. The reason why they were in search of alternatives was the U.S.’ unilaterally expelling NATO-ally Turkey from the program over Ankara purchasing Russian-made S-400 air defense systems.

From then on, new manufacturers have been required, while Turkey continued to produce the components after the country was unfairly expelled from the program. Ankara has reiterated various times that the removal was illegal and uniliteral, demanding a fair approach and saying that it is open to negotiations with Washington.

The Block 4 modernization program, which officially began in 2018, envisages the U.S. Defense Department’s upgrading of the Lockheed jet’s hardware and software, adding new weapons capabilities and computing systems. The GAO report said software defects weren’t being caught before they were installed on the aircraft and that it was taking longer than planned to develop each new capability.

The fact that the Block 4 upgrade program has now cost at least $14 billion makes it more expensive than many Army, Navy and Air Force acquisition programs.

Among the reasons for the $1.9 billion excess cost in the GAO report include: