(Reuters) -Lockheed Martin has delivered 72 F-35 jets to the U.S. government as of May 1, after several months of delay due to late software improvements, Bloomberg News reported on Monday, citing the Pentagon's program office.

The upgrade, known as Technology Refresh 3 (TR-3), aims to give the stealth fighter software and hardware improvements, including better displays and processing power. It has been weighed down by delays, prompting the Pentagon to withhold $5 million per jet since last year.

That number was reduced by about $1.2 million per plane in January as the defense contractor was considered to be making some progress with the upgrade, according to the report.

Funds will continue to be withheld for new production aircraft, the report said, adding that a majority of funds remain to be withheld and "will continue to be released incrementally" into next year.

The Pentagon's program office did not immediately respond to a Reuters request for comment, while Lockheed Martin declined to comment ahead of its second-quarter earnings next week.

The F-35 program is vital to Lockheed, as it contributes about 30% of its revenue. The world's biggest defense contractor is also likely to take a hit from fewer F-35's requested by the Trump administration in the fiscal 2026 budget.