With Greece, Diaspora groups and some members of the US Congress anxious over Turkish provocations in the Aegean and East Mediterranean, Turkey’s President Recep Tayyip Erdogan said he will go ahead with a purchase of a Russian S-400 missile defense system.

That’s in defiance of a United States warning it could jeopardize Turkey’s hopes of also buying American-made F-35 fighter jets and worries the Russian deal could undermine the defenses of NATO, to which Greece and Turkey belong.

There’s also concern that if Turkey has both the F-35s and the S-400 that it could have a strong strategic advantage over Greece with fears there could be an accidental – or planned – conflict with Erdogan ramping up bellicose talk and sending energy research vessels and warships off Cyprus to look for oil and gas in areas where foreign companies are licensed to drill.

Turkey regularly sends F-16 fighter jets and warships into Greek and Cypriot waters and past Greek islands with no intervention from NATO as the defense alliance’s chief, Jens Stoltenberg, sid he wants no part of any trouble between the alleged allies.

“There is an agreement. We have determination. It is out of the question to take a step back,” the Anadolou News Agency quoted Turkish President Recep Tayyip Erdogan as telling reporters in Constantinople on June 4.

The US said the S-400 is incompatible with NATO systems and that Russian hardware deployed in NATO-member Turkey would allow Moscow access to sensitive defense intelligence concerning the alliance.

Erdogan said Turkey made several failed attempts to purchase the American-made Patriot defense system, adding that he would still be open to the option if “there is as positive an offer as Russia made,” which could lead to US sanctions.

Bloomberg last week reported that Turkey is pondering the possibility of deploying the system along its southern coast, close to the area where it has military vessels accompanying drillships looking for gas deposits in the Eastern Mediterranean.