With growing indications the United States is siding with Greece as tension grows in the Aegean and East Mediterranean, Turkish fighter pilots training in F-35 jets has been stopped, as could the potential sale of the advanced weapons aircraft to Turkey.

Washington is upset that Turkish President Recep Tayyip Erdogan is planning to go ahead and purchase a Russian S-400 missile defense system that could compromise NATO, the defense alliance to which Greece and Turkey belong.

The US wants Turkey to buy the American-made Patriot missile defenses but Erdogan is adamant he will go ahead with the Russian system even though it’s coming from one of NATO’s ideological enemies.

The US State Department has given Turkey until July 31 to call off the sale of the Russian system or lose any chance of buying the F-35’s and the end of training for the Turkish pilots, who were also cut off from access to the jet’s restricted information was seen as a further warning.

The developments were reported by Foreign Policy magazine and came as fears are growing of an accidental conflict – or deliberate – in the seas around Greece and also Cyprus, where Erdogan has sent two energy research vessels and a warship into the island’s Exclusive Economic Zone (EEZ) to hunt for oil and gas.

That’s near where the legitimate government – Turkey occupies the northern third it seized in an unlawful 1974 invasion – has licensed foreign energy companies, including US’ ExxonMobil to also drill, with the company reporting a major gas find.

Turkey regularly violates Greek airspace and waters with fighter jets and warships and there were worries that if Turkey acquires both the Russian missile defenses and the F-35s that it could put Greek fighter pilots at a distinct strategic air disadvantage.

The US military barred Turkish pilots to restricted information amid concerns that “continuing to allow the Turkish pilots access to the F-35’s most sensitive data – instruction manuals, for example – after the July 31 deadline was imposed would provide them an opportunity to take classified information out of the secure space,” an unnamed official told the magazine.

“Without a change in Turkish policy, we will continue to work closely with our Turkish ally on winding down their participation in the F-35 program,” Pentagon spokesman Lt. Col. Mike Andrews told the magazine in a sideways slap as the US has a military presence in Turkey too.

New York Democratic Congressman Eliot Engel, Chairman of the US House of Representatives’ Foreign Affairs Committee, said, “if Turkey follows through with the acquisition of the S-400, it would trigger sanctions in Congress overwhelmingly, bipartisanly passed two years ago.”

Erdogan is “cozying up to Russian dictator Vladimir Putin, putting the security interests of the United States and the NATO alliance at risk,” Engel said, warning that plans for S-400 system it would be doing “lasting damage to our historic bilateral relationship,” said Kathimerini.

“If Turkey decides to follow through… they must not be able to get American F-35s,” he added. “We rarely see it in foreign affairs, but this is simply a black-and-white issue; there’s no middle ground.”