Washington, Sep 17 (IANS): The US Air Force said that it has grounded 10 of its F-35 Joint Strike Fighters, just over a month after they were declared "combat ready".

The decision affecting the most expensive weapons system ever was made "due to the discovery of peeling and crumbling insulation in avionics cooling lines inside the fuel tanks," the Air Force said in a statement on Friday, describing the action as a "temporary pause in flight operations".

The faulty cooling lines affected a total of 57 aircraft, the statement said. Only 15 of those planes had been fielded with the remainder still on the production line and will be fixed there, CNN reported.

The plane's manufacturer, Lockheed Martin, has delivered 108 F-35As. The Air Force plans to buy 1,763 of the jets.

Of the 15 grounded aircraft already in the field, 10 had been declared combat ready, one was being used in testing, and the final four were for training, with two of those four training aircraft belonging to the Royal Norwegian Air Force.

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

Representatives from the Air Force and Lockheed Martin's F-35 programme told CNN that the defective cooling lines were not found in all the aircraft. They said the use of the faulty part was limited to one sub-contractor that did not work on all the planes.

"Safety is always our first consideration and Lockheed Martin is committed to resolving this issue as quickly as possible to return jets to flying status," Michael Rein, a spokesman for Lockheed Martin, told CNN in a statement.

The F-35 jet has been described as the most expensive weapons system in history with a programme cost estimated to be $400 billion.

The F-35A is the Air Force's version of the jet, the US Marines and Navy will also have their own F-35 variants.