Acting US Defence Secretary Patrick Shanahan said on 31 May that he is unaware of plans to create a joint working group with Turkey on the S-400 issue, which were reported on earlier in the day.
"I haven't heard of the joint study group", he said during the Asia Security Summit in Singapore.
Shanahan reiterated his stance on the problem with Turkey's acquisition of Russian air defence systems, arguing that the S-400 is incompatible with the F-35 and that therefore the US shouldn't sell the latter to Ankara.
"The S-400 is designed to shoot down the F-35. They are natural enemies, that's the fundamental reason", he said.
US President Donald Trump has accepted an offer by his Turkish colleague Recep Tayyip Erdogan to create a working group on Russian S-400 air defence systems, which will be procured by Ankara this summer, Turkish NTV reported.
"Trump reacted positively to Erdogan's proposal to create a working group on S-400", the media's source said.
Erdogan earlier offered the US to form a joint working group to iron out Washington's concerns regarding the system being able to reveal the F-35's vulnerabilities to Moscow. He raised the issue of the working group on S-400 in a telephone conversation with Trump on Wednesday.
The US has been pressuring Turkey into abandoning the S-400 deal with Russia ever since it was inked in December 2017. Washington claims that the system will be incompatible with NATO equipment and that it will endanger F-35s, if they are operated by Turkey along with S-400s.
Under this pretext, Washington has repeatedly threatened to halt the procurement of F-35s by Ankara and even mulled moving the production of some of the jet's parts from Turkey to Europe. What is more, US officials have warned Ankara that it could fall under US sanctions over the deal with Moscow and may even lose NATO member status if it doesn't back off.
Ankara slammed Washington's actions, while at the same time assuring that S-400 won't pose any threat to NATO or F-35s. Erdogan has repeatedly insisted that Turkey will not abandon the deal and said that the deliveries of Russian defence systems will be made on schedule.