Turkey and the United States are at odds over Ankara's decision to buy the S-400s, which cannot be integrated into NATO systems. Washington says the move would jeopardise Ankara's role in building Lockheed Martin F-35 fighter jets, which it says would be compromised by the S-400s.

Speaking to reporters late on Tuesday, Akar said that Turkey was fulfilling its reponsibilities in the F-35 project and that Ankara expected the programme to continue as planned.

He said talks with US officials were still underway.

He also said Turkey was evaluating a US offer to purchase the Raytheon Co. Patriot systems, adding that Ankara and Washington were working on price, technology transfer, joint production issues on the latest offer from US officials.