US Air Force GROUNDS $400billion F-35 fighter jets due to 'faulty cooling lines' - a month after declaring them 'combat ready'

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The US Air Force is struggling to get the world's most expensive weapon off the ground.

Just a month after declaring the F-35 'combat ready,' officials have now grounded 10 of the fighter jets.

US Air Force officials have grounded a number of F-35 aircraft 'due to the discovery of peeling and crumbling insulation in avionics cooling lines inside the fuel tanks'

The temporary pause in flight operations came 'due to the discovery of peeling and crumbling insulation in avionics cooling lines inside the fuel tanks,' the Air Force said in a statement Friday. 

The problem affects 57 of the 108 F-35s that manufacturer Lockheed Martin has delivered.

This number includes 15 aircraft that were already in the field — and 10 that had been declared combat ready. 

'While nearing completion, the F-35 is still in development and challenges are to be expected,' the Air Force said in a statement.

The problem affects 57 of the 108 F-35s that manufacturer Lockheed Martin has delivered

Officials say the F-35 will give the U.S. military the ability to detect enemy aircraft and other threats far beyond current ranges, allowing the jets to strike targets and disappear long before they are detected.

With a price tag of $400 billion, the aircraft is considered the most expensive weapons system in history. 

The U.S. Air Force plans to buy a total of 1,763 F-35A conventional takeoff and landing jets in coming years and will operate the largest F-35 fleet in the world.

Air Force General Herbert Carlisle, commander of Air Combat Command, said work to upgrade the jet would continue in areas such as software, making the displays more intuitive and boosting the ability to share information between aircraft. 

With a price tag of $400 billion, the aircraft is considered the most expensive weapons system in history

Lockheed is building three models of the F-35 Lightning II for the U.S. military and 10 countries that have already ordered the jets: Britain, Australia, Norway, Italy, Turkey, Denmark, the Netherlands, Israel, South Korea and Japan.

The Pentagon's F-35 program office said it remained in negotiations with Lockheed over long-delayed contracts for the next two batches of F-35 jets, deals worth about $15 billion.

The program, launched in 2001, has made strides in recent years after huge cost overruns and technical problems that sent the project's cost up nearly 70 percent.

Problems with the fighter jet included issues with the radar software and increased risk of neck injury to lower-weight pilots when they ejected from the aircraft. 

The U.S. Air Force plans to buy a total of 1,763 F-35A conventional takeoff and landing jets in coming years and will operate the largest F-35 fleet in the world

Industry and U.S. defense officials say they are working hard to continue driving down the cost of the new warplanes to $85 million per plane by 2019, as well as the cost of operating them.

Sen, John McCain, the Republican chairman of the Armed Services Committee, said he welcomed the announcement but made clear he intended to keep a close eye on the hugely expensive program.

'The Senate Armed Services Committee will continue to exercise rigorous oversight of the Joint Strike Fighter program's long-delayed System Development and Demonstration phase as well as the start of the operational test and evaluation phase,' McCain said in a statement.

The program, launched in 2001, has made strides in recent years after huge cost overruns and technical problems that sent the project's cost up nearly 70 percent

To become battle ready, at least a dozen individual F-35 must demonstrate their ability to drop bombs and shoot down other planes.

Each jet must be upgraded to a specific software package, and plugged into the complex logistics cloud that manages maintenance.

The F-35 project office had previously set an Aug. 1 target date — but the project has been plagued with delays.

The F-35 Joint Strike Fighter's record on cost, schedule and performance has been a scandal and a tragedy, McCain told senior Pentagon officials earlier this year.