The US envoy in NATO said if Turkey purchases Russian S-400 defense systems, the country won’t be allowed to use the F-35 jets.

Reuters reported that US Envoy Bailey Hutchison said, “There will be a separation with the F-35 system. For the alliance, the presence of the Russian system and the F-35 program being affected or destabilized is not possible.”

Turkish President Recep Tayyip Erdogan spoke during the weekend and said the S-400 systems would start being delivered in the first week of July despite disapproval from the NATO and the US.

The US has been preparing for sanctions in the rising tensions between the US and Turkey regarding the S-400 systems.

Pentagon issued a statement on June 11 and announced that they stopped training for Turkish F-35 pilots.

Pentagon Spokesperson Mike Andrews said, “Our Department is aware that the Turkish pilots in the Luke Air Base are not flying anymore.”

US Acting Defense Secretary Patrick Shanahan had sent a letter to Turkish Defense Minister Hulusi Akar and said if the country goes through with the purchase of the S-400 missile defense systems they would face “serious sanctions”.

Several parts of the F-35 jets, manufactured by the American company Lockheed Martin, are sourced from Turkey. In April, American officials warned their Turkish counterparts that the delivery of equipment related to the F-35 project had been stopped, and that the dispatch of all future training and F-35 related equipment had been cancelled.

The US and Turkey’s other NATO allies who have F-35s are concerned for the S-400s. The concerns are centered around the possibility of Russia learning how to track and identify the jets with the missile defense system and that the jet will become more vulnerable against Russian made weapons.