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TANAGRA, Greece (AP) — Greece’s air force on Monday took delivery of a first pair of upgraded F-16 military jets under a $1.5 billion program to modernize its fighter fleet amid increasing tensions with neighboring Turkey.

The two F-16s presented at the Tanagra airbase northwest of Athens are the first of 83 to be refitted with advanced electronics, radar and weapons capabilities by late 2027 by Greece’s Hellenic Aerospace Industry, in coordination with U.S. manufacturer Lockheed Martin.

The head of Greece’s joint chiefs of staff, Gen. Constantinos Floros, said the program’s successful and timely completion “is an issue of the highest national importance.”

“Any potential aggressor will have to think twice or thrice before trying their luck,” once the upgrade is completed, he said at Monday’s presentation.

Relations with historic regional rival Turkey have hit a new low following repeated, thinly-veiled threats from Turkish President Recep Tayyip Erdogan that his country might invade Greece’s eastern Aegean Sea islands.

Turkey claims the islands have been illegally militarized. Greece says it needs to defend them against potential attack. The two NATO allies are also at odds over offshore gas and oil rights, and their fighter jets routinely intercept each other or engage in simulated dogfights over the Aegean.

Asked Sunday by The Associated Press whether the recent escalation in rhetoric from Turkey could be the prelude to an armed conflict, Greek Prime Minister Kyriakos Mitsotakis replied negatively.

“I don’t believe this will ever happen,” he said. “And if, God forbid, it happened, Turkey would receive an absolutely devastating response.”

On Monday, Mitsotakis said Athens “always keeps a window open to dialogue and discussion” with Turkey. He spoke in Paris ahead of talks with French President Emmanuel Macron.

Following years of forced savings during the 2010-2018 financial crisis, Greece has embarked on a multibillion-dollar spending spree to boost its armed forces. It has bought or ordered French Rafale fighter jets and FDI frigates, and plans to purchase F-35 fighters from the U.S.

The F-16s, developed in the 1970s, are the workhorse of Greece’s air force. It acquired a first batch of 40 in 1989, and another 130 over the years. The latest upgrade will bring the 83 planes to the Block 72 variant that’s the most advanced F-16 version in service in Europe.

Another four F-16s will be upgraded by the end of this year.