The defense establishment is reconsidering the purchase of American F-35 fighter jets due to the unexpected high cost and disagreements with the manufacturer regarding the installation of Israeli systems in the planes.

"This is undoubtedly the ideal fighter plane, and we'd like to have it very much, but not at any price," a senior defense establishment official said.

A defense establishment evaluation concludes that the Israel Air Force can maintain its operational and technological supremacy even without the advanced fighter aircraft. This could be achieved by upgrading the IAF's F-15 and F-16 aircraft and buying their advanced models.

The Lockheed Martin F-35 is a fifth-generation, single-seat, single-engine, stealth-capable military strike fighter, which can evade radar and anti-aircraft missile systems. Senior Air Force officers insist on purchasing it to deal with future battle threats.

Israel is interested in signing the preliminary contract to buy the planes by the end of the year. The first 25 planes are expected to arrive from 2014, and Israel is planning to buy 50 more planes in the future.

The Pentagon and Congress approved the deal, estimated at $15 billion, last autumn. However, disagreements between the Israeli Defense Ministry and Lockheed Martin is holding up the negotiations, which have reached an advanced stage.

The main argument is about the system structure integrating all the avionic systems. This makes it difficult to meet IAF demands that unique communications, controls and special electronic systems, at the very least, be installed, and that the plane be modified to carry Israeli-made armaments.

Because of the F-35's structure, which places missiles, bombs and electronic devices inside the plane, no system can be installed externally and considerable internal modifications would be required.

These modifications push up the aircraft's price by at least 25 percent. In 2002, Lockheed Martin cited the cost of one F-35 at some $47 million, but now the official price is about $80 million, in part reflecting the dollar's devaluation. The modifications Israel is demanding raise the price tag to over $100 million.

One Pentagon estimate quoted the cost of 75 F-35s in addition to spare parts, engines and a comprehensive support system at $15 billion. Senior defense establishment officials say such prices are prohibitive.

The F-35 purchase deal is the largest in Israel's history. The defense establishment is afraid to mortgage most of the American defense aid earmarked for purchases in the United States for this deal, especially when the Defense Ministry's budget is expected to be cut.

If the budget is slashed, the defense establishment will have to use some of the American aid money to buy ammunition and bombs (to renew the supplies diminished by the Second Lebanon War and Operation Cast Lead) in the U.S. instead of buying them in Israel.

IAF commanders praise the F-35's operational abilities and say that having these planes would generate a deterrence effect similar to the one created by purchasing the F-15 in 1976.

The IAF had begun building a complete aerial conception for the future decades. The air force says that only the F-35 can deal with advanced S-300 anti-aircraft systems that Russia may supply Iran with soon, and in any case some of the earlier F-15 and F-16 aircraft models, which arrived at the end of the '70s, will have to be replaced soon.

Eight years ago Israel could have joined the development of the F-35 for a $100 million investment. But the defense establishment believed at the time that, in any case, Israel would be able to buy the plane in the future and integrate Israeli systems into it.

Last year Defense Minister Ehud Barak approved purchasing the first 25 F-35 aircraft as part of the IDF's multi-year plan. At the same time the Pentagon approved the sale and option for 50 additional aircraft.