Israeli officials are concerned about a deal to provide F-35 stealth fighter jets to Turkey, and the issue is currently being discussed with the United States. A senior Israeli defense official says Israel would like to remain the only country in the Middle East with the F-35 in order to keep its military qualitative edge and out of fear that details about its capabilities would leak to neighboring countries.

Talks between Israel and the United States are centered, among other things, around software developed by the Americans that allows the "upgrading" of F-35 capabilities. Sources in Washington confirmed to Haaretz that the issue is currently "part of the negotiations" relating to the F-35 deal.

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Officially, Israel denies it is holding discussions over the deal, under which Turkey is slated to receive 100 F-35 planes, and that it is only closely monitoring developments. Sources in the Israeli defense establishment predict it will be very difficult to cancel the deal, because Turkey was one of the countries that invested in the plane's development.

The Israel Air Force is scheduled to receive the software that will improve the plane's performance in July, and Israel is worried that Turkey will also receive it. One possibility being discussed is providing Turkey the plane without the software, thus keeping Israel's edge.

Meanwhile, criticism of the deal is growing in Congress. A number of lawmakers are pushing a bill that would halt the implementation of the agreement to sell Turkey the planes in light of recent events in the country.

"Our concern is that Turkey is going through a very dramatic transition as a country," says Senator James Lankford, a Republican from Oklahoma and one of the leading voices on Capitol Hill against the F-35 deal.

"Turkey has gone a long way from being a NATO ally and an important partner in working against terrorism, to the situation today, where it is holding an American citizen as a bargaining chip," he told Haaretz, referring to Andrew Brunson, a pastor under arrest in Turkey since last year. "This is not the behavior of an ally," Lankford said.

This incident, Lankford says, is indicative of a broader change taking place in Turkey, in which the country is becoming less reliable as a U.S. ally.

"There's tremendous amount of frustration," he explains. "They're arresting journalists, pastors, teachers, political opponents of the government. This is a country that is going through a very consequential transition, and I ask myself, why are we giving our best military technology to someone who is going through such a transition?

"My concern is - they're a NATO ally, they have been a good partner for years, but if we don't know what the country is going to be like in a few years, we should withhold this resource from them," Lankford says. Comparing Turkey and Israel, he explains that the United States has "no hesitation with Israel. When we give them the F-35 or other military equipment, we know how they will use it. We know what they will and won't do. I'm not sure we can say the same about Turkey."

He adds that while he would like to "keep Turkey as an ally," recent internal changes in Turkey and disagreements with the United Sates on foreign policy issues – including President Erdogan's display of hostility toward Israel – should make the U.S. "take a pause" and reevaluate the F-35 deal, and potentially other forms of military cooperation as well.

A spokesperson for AIPAC, the pro-Israel lobby, said it hasn't taken a position on Lankford's bill. The Israeli Embassy in Washington has also kept silent on the issue. But a foreign policy adviser to a senator involved in the discussions told Haaretz that lawmakers who asked Israeli officials about the deal heard reservations.

"No one here has any doubt that Israel prefers to stay the only country in the region that has these attack capabilities," he said. "The Israelis know how to make that clear, in their own ways."