WASHINGTON (Reuters) - The U.S. Air Force has grounded 13 Lockheed Martin Corp F-35A Lightning II aircraft after discovering peeling and crumbling insulation in avionics cooling lines inside the fuel tanks, an Air Force spokeswoman said on Friday.

The disclosure was made less than two months after the Air Force announced that an initial squadron of the F-35A stealth fighters were ready for combat, marking a major milestone for the $379 billion program, the Pentagon's largest weapons project.

"The issue was discovered during depot modification of an F-35A and affects a total of 57 aircraft," Air Force spokeswoman Ann Stefanek said in a statement.

The planes included 15 aircraft deployed in bases in Utah, Arizona and Nevada, Stefanek said, adding that 13 belong to the United States and two belong to another country.

The other 42 affected planes are still under production, she said.

It was not immediately known if more serious problems could arise.

Engineers of the F-35 Joint Program Office and Lockheed Martin and Air Force maintenance teams at Hill Air Force Base in Utah are developing plans to fix the problem, the spokeswoman said.

"The Air Force is also working with units to mitigate the impact on operations, training, and readiness," Stefanek said.

(Reporting by Idrees Ali; Writing by Mohammad Zargham; Editing by Jeffrey Benkoe)