The Croatian military is on the verge of selecting Israel’s used F-16 jets to take the place of its dwindling supply of Soviet MiG-21s, just six of which are estimated to be operational. The fleet of 21 60-year-old MiG-21s is in dire need of replacement.

Croatia invited the US, Israel, South Korea, Sweden and Greece to participate in the competition to supply the next wave of fighter aircraft last July. Zagreb has eyed new or used F-16 Fighting Falcons from Israel, the US or Greece, South Korea’s FA-50s and Saab’s JAS-39 Gripens.

Israeli Prime Minister Benjamin Netanyahu and Croatian Prime Minister Andrej Plenkovic reached an agreement “to push forward” on the deal, signaling Netanyahu’s F-16s may be close to winning the contract, the Jerusalem Post reported Friday.

“This development is another expression of the deep ties between the two countries,” Netanyahu’s office said in a statement. The deal could be worth up to $500 million, but its potential terms have not been finalized.

The Israeli Air Force retired its F-16A/B fleet in late 2016 but still flies heavily modified F-16C/D aircraft. It’s not clear what composition of planes Netanyahu has offered to sell to Zagreb, but Croatian media reported that Sweden had sweetened its deal by offering to provide the more advanced F-16C/D jets.

Speaking with Stars and Stripes, a Croatian military analyst said no more than six of the MiG-21s are airworthy. “It was a great plane during Croatia’s war of independence” from 1991 to 1995, but “it really can’t hold its own against modern fighters or anti-aircraft defenses,” analyst Denis Kuljist told the military news outlet January 18.

The MiG-21, NATO reporting name Fishbed, has been one of the most widely used fighter jets in military history and achieved the mark of being the most-produced supersonic aircraft in history.