Israeli military planners say they are increasingly worried that problems in Lockheed Martin's F-35 Joint Strike Fighter program will mean that delivery of 20 of the stealthy jets ordered in October 2010 for $2.75 billion will be delayed.

If the delivery timetable is pushed back to later than 2017, senior military officers say the Israeli air force could consider acquiring new Boeing F-15 Eagles or Lockheed F-16 Fighting Falcons to fill the gap.

At present, the first of the 20 F-35s -- enough for one squadron -- is scheduled to arrive in Israel in 2015 with delivery completion in 2016. Pilots are to undergo training with the U.S. Air Force starting in 2014.

Until a few weeks ago, Israeli officials were confident that the first JSF squadron would be delivered on time, despite problems that have delayed the project for at least three years.

This optimism flew in the face of the technical setbacks that have plagued the development of what is billed as the world's most advanced fighter and even suggestions in Congress the high-profile project be scrapped because of massive cost overruns.

That confidence was undermined after the Pentagon's director of the F-35 program, U.S. Navy Vice Adm. David Venlet, urged slowing production because of cracks and "hot spots" that are showing up during fatigue testing and analysis.

"The analyzed hot spots that have arisen in the last 12 months or so in the program have surprised us at the amount of change and the cost," Venlet told the Web publication AOL Defense.

"Most of them are little ones. But when you bundle them, all up and package them, and look at where they are in the airplane and how hard they are to get at after you buy the jet, the cost burden of that is what sucks the wind out of your lungs," he said.

"I believe it's wise to sort of temper production for a while here, until we get some of these heavy years of learning under our belt and get that managed right."

The Israeli Defense Ministry has budgeted the purchase of more F-35s for a second squadron under the military's new multiyear plan that begins in early 2012.

Ultimately, the air force wants three F-35 squadrons to form the spearhead of its strike force -- some 75 aircraft. The Pentagon has already approved the sale to Israel of 55 of the single-engine jets at dates yet unspecified.

The Israelis say they need the F-35 to maintain their long-held air superiority in the Middle East, and in a wider context, the Jewish state's technological edge over its regional adversaries, Iran in particular.

Israel's main problem is that senior U.S. air force officers said in testimony before Congress in November that the F-35's development problems mean it's not likely to become operational with U.S. forces until 2018 -- two years later than planned.

Aviation Week reported recently that the U.S. Air Force plans to upgrade more than 300 Lockheed Martin F-16 fighters and probably some Boeing F-15s as well to fill the operational gap caused by the F-35 delays.

Despite the Israeli Defense Ministry's public position that procurement plans won't be affected by the F-35 delays, senior officers say additional delays could lead to the acquisition of new-model F-15s and F-16s to maintain Israel's qualitative edge in the air until the JSFs do actually arrive.

Boeing unveiled the F-15SE Silent Eagle in March 2010. It reportedly has a stealth capability that can evade radars carried by enemy aircraft but not ground-based radar systems.

The Israeli air force currently operates 25 customized F-15I -- for Israel -- Ra'am aircraft, and 101 F-16I Sufa jets that form its strategic spearhead and would be the squadrons deployed in any strike against Iran.

The F-15I has the longest range in the Israeli air force.

The Israelis paid close attention when Boeing launched the F-15SE in July 2010 at its St. Louis headquarters.

Another problem for the Israelis is the mounting cost of the F-35.

When Israel bought into the JSF program in 2002, the price tag was $69 million per aircraft. But that's constantly rising and could go as high as $150 million because of cost overruns.

The F-15SE reportedly costs around $100 million per aircraft.