ANKARA - Turkish President Recep Tayyip Erdogan warned on Friday that Ankara might reconsider buying Boeing aircraft from the United States if it follows through on its threat to impose sanctions over the acquisition of Russian S-400 air defence missile systems.

"We are buying advanced Boeing aircraft. We are good customers. But if this [sanctions] happens, we may reconsider the purchases", Erdogan said in a speech in Ankara.

Turkey defied US calls to abandon the S-400 deal with Russia and received first shipments of the technology. Washington said it began unwinding Turkey from their joint F-35 jet program in response and did not rule out other penalties.

Turkey turned to Russia after years of feet-dragging in Washington over its request to buy Patriot missile systems. President Donald Trump said in June that the previous US administration had treated Turkey unfairly but US media have since reported that Congress had stepped up pressure on him to sanction his NATO ally.

Russia completed shipping the first portion of S-400 air defence systems to Turkey on 25 July under a contract that was inked in December 2017. The US pressured Turkey hard to drop the deal, offering to sell Patriot missile systems instead – the ones that Washington initially refused to sell Ankara. Washington has cited alleged the incompatibility of S-400s with NATO systems and their threat to F-35 jets, as a cause of concern.