The delivery schedule for Russia's S-400 defense systems to Turkey is continuing as planned, Turkish Foreign Ministry spokesperson Hami Aksoy said Friday, dismissing reports that Ankara was evaluating a delay in response to US concerns.

Turkey's purchase of the Russian systems has strained its already tense ties with the US, which says the S-400s are not compatible with the alliance's defense network and pose a threat to F-35 stealth fighter jets which Turkey is due to receive.

Ankara has since proposed forming a working group to assess the US concerns, but has yet to hear back from Washington on that proposal. The US has warned of sanctions if Turkey presses ahead with the deal.

Speaking with reporters in Singapore, acting US Defense Secretary Patrick Shanahan said while he was not aware of a working group, he had spoken with his Turkish counterpart in the last week and they were making progress on discussions.

On Tuesday, sources said that the US was seriously considering suspending training for Turkish pilots on the Lockheed Martin F-35 jets over Ankara's decision to continue with the S-400 deal.

Asked whether a decision had been made to send Turkish pilots back, Shanahan said Turkish Defense Minister Hulusi Akar wanted to meet to discuss the path forward. He added that his position had not changed with respect to Turkey's purchase of the S-400 and Ankara should not get the F-35 if they go ahead with it. 

Turkey has said that as a NATO member it poses no threat to the US and the sanctions should not apply.