The United States has not moved to create a joint working group to assess its concerns regarding Turkey’s purchase of the Russian S-400 missile defence systems, the head of the Turkish Defence Industries Directorate said on Monday.


The two NATO allies have sparred publicly for months over Turkey’s order for the S-400s, which are not compatible with the transatlantic alliance’s systems.


Washington says the S-400s pose a threat to the Lockheed Martin Corp F-35 fighters and has warned of potential U.S. sanctions if Ankara presses on with the deal.


Turkey, one of the core partners in the F-35 programme and a prospective buyer, has said the S-400s will not impact the jets and has proposed to Washington forming a joint working group to assess U.S. concerns.


Speaking to reporters after an event in Ankara, Ismail Demir, who heads the state defence industry institution, said Turkey was ready to discuss Washington’s concerns.


“If the source of the concerns is a technical worry stemming from the S-400s being located in Turkey, we have said repeatedly that we are ready to discuss this,” Demir said.


“However, the other side (United States) has not taken any steps to form the technical team and discuss this.”


Demir’s comments come after U.S. Defense Secretary Patrick Shanahan last week sent a letter to his Turkish counterpart, in which he outlined how Turkey would be pulled out of the F-35 programme if it pressed on with the S-400 deal.


Demir said the Defence Ministry and his directorate were working on sending a response soon.


PATRIOT OFFER


While Turkey has dismissed U.S. warnings, saying the S-400s were a “done deal”, Washington has said discussions are taking place with Ankara on potentially selling Turkey rival Raytheon Co Patriot missile defences.


President Tayyip Erdogan said on Tuesday that it was “out of the question” for Turkey to back away from its deal with Moscow and added that the United States had not “given us an offer as good as the S-400s” on the Patriot systems.


On Monday, Demir said Turkey responded to the latest U.S. Patriot offer and was waiting for a response, but did not elaborate further.


In a sign of escalating tensions, sources said that Washington has decided to stop accepting any more Turkish pilots who planned to train in the United States on the F-35 jets.


Ties between Ankara and Washington have already been tense over a host of issues, including conflicting strategy in Syria, Iran sanctions and the detention of U.S. consular staff in Turkey. But removing Turkey from the F-35 programme would be one of the most significant ruptures of recent times.