WASHINGTON — Lockheed Martin has finished building fewer than half of the upgraded F-35 Joint Strike Fighters it promised this year, as slow production of key parts are holding up multiple new jets, the Pentagon’s top officer in charge of the program told lawmakers Tuesday.

Lt. Gen. Michael Schmidt, the F-35′s program executive officer, said at a House Armed Services tactical air and land forces subcommittee hearing Lockheed Martin is under contract to deliver 52 jets enabled with improvements known as Technology Refresh 3 by the end of this year. The company has finished construction on 21 of those.

But production on a handful of key components needed for the TR-3 hardware has ramped up slower than expected, Schmidt said, and the rest of the newest jets are still sitting at Lockheed Martin’s factory in Fort Worth, Texas, waiting for those parts. Schmidt declined to describe these parts to reporters after the hearing.

“The hardware is good,” Schmidt told reporters after the hearing. “It’s the rate at which they’re producing them to meet our production and retrofit needs. It is literally a small number of components, but I need all the components.”

TR-3 upgrades include hardware and software improvements to the F-35, such as better displays, computer memory, and processing power. They are needed for a more expansive upgrade called Block 4, which will improve the F-35′s weapons capacity, target recognition, and electronic warfare, among other features.

The Pentagon and Lockheed Martin sought to have TR-3 ready by April 2023. That date has now slipped considerably, largely because of persistent software and integration issues. Schmidt said Tuesday TR-3 might be done by mid-spring, though he was not confident in that deadline. The government and Lockheed Martin said previously it could be as late as next June.

The government is not accepting the newly built F-35s, because the military has not been able to carry out the test flights necessary to make sure they work properly.

Subcommittee chairman Rep. Rob Wittman, R-Va., said in the hearing that while the F-35 is a “technological marvel,” its repeated delays in fielding capabilities are disturbing.

Schmidt said part of the problem has been with underperforming labs the program has used to try to test TR-3, and that the program is trying to create more capacity in its labs to handle the greater capabilities intended for the F-35.

“Our labs are not properly representing the flight environment, and there’s way too much discovery happening in flight test,” Schmidt said.

He told reporters Lockheed “is paying a significant price” for its inability to meet its contractual requirements on the TR-3 jets, though he would not detail these penalties.

“Lockheed is very much incentivized to deliver,” he said.

Lockheed Martin did not immediately respond to a request for comment.

The slow component production right now is only affecting new jets, he said, because the F-35 program hasn’t started working on retrofitting older jets.

But if the production issues aren’t ironed out, that could lead to more problems, he said.

“They’ve got a little time here to ramp up, but they need to do it quickly,” Schmidt said. “It’s not good to get to a certain point in the production line where this part needs to go into that part, and that part’s not there.”