RAF FAIRFORD, England (Reuters) – The United States has resumed taking deliveries of F-35 jets after an interim upgrade following a months-long hiatus caused by software delays and will withhold some payments until remaining upgrades are completed, senior officials said on Saturday.

Lockheed Martin (NYSE:) and key suppliers are modernizing the planes under a program called Technology Refresh 3 (TR-3) that aims to improve displays and processing power.

“We’re starting to take deliveries of the TR-3… It’s a scaled-down version… and it doesn’t have all the features we want, but it’s advanced enough that we can start taking deliveries,” Air Force Secretary Frank Kendall said.

The TR-3 includes both hardware and software improvements and is considered the linchpin of a broader stealth aircraft upgrade known as Block 4.

“The hardware seems to be fine but the software is faulty,” Kendall told reporters at the Royal International Air Tattoo in the UK.

“We’re now at a point where we can accept the aircraft, understanding that there are additional software increments that need to be made to get them to the state that we need them to be in.”

Andrew Hunter, assistant secretary of the Air Force for acquisition, technology and logistics, said the decision to resume deliveries without a full software update was made in part to avoid delays in other functions.

“Waiting and waiting and waiting for these capabilities to be finalized that weren’t ready yet is actually holding up progress on some of the later capabilities that we really need,” he said.

Hunter confirmed the Pentagon will continue to hold back final payments for each aircraft, but did not provide a specific amount.

So far, a final payment of about $7 million has been withheld for each aircraft, worth approximately $100 million each.

“Not everything we contracted for was delivered … We won’t pay for what we didn’t receive,” Hunter said.

But he added that efforts to complete the TR-3 program have helped improve cooperation among companies involved in the world’s largest defense program.

“We will work hard to ensure that the process of tearing down these barriers continues so that Unit 4 is completed when it is needed,” he said.