ZAGREB, Croatia (AP) - A Croatian deal to buy 12 used F-16 jets from Israel worth $500 million could be in jeopardy if the US does not approve of the sale, officials said Wednesday.

The tentative deal to purchase the upgraded F-16 Barak fighter jets from Israel was made earlier this year pending a U.S. approval allowing Israel to sell the American-made jets to a third party.

Israel has upgraded the jets with sophisticated electronic systems - crucial in Croatia’s decision to buy the planes from Israel rather than from the U.S.

The U.S. State Department is now hinting that the upgrades should be removed before the jets can be sold.

Croatian Prime Minister Andrej Plenkovic said “we will either buy the planes that were originally offered (by Israel) or the tender will be declared null and void, there is no dilemma about it.”

“The burden of obtaining consent from the United States for the delivery of the planes was taken on by the Israeli side,” Plenkovic said. “At this moment, this is a matter of relations between Israel and the United States.”

Relations between the Trump administration and Israel have been very close, particularly on defense issues.

The State Department said that it on Tuesday “formally notified Congress of this transfer case.” It added: “Although certain details are pending, the department would like to see cooperation from all parties involved to come to a successful resolution.”

The deal is Croatia’s largest single military purchase since it split from the Yugoslav federation in the 1991-95 war.

NATO member Croatia faces a mini arms race with Russian ally Serbia, which recently received six used Russian MiG-29 fighter jets.

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