Turkey's ongoing negotiations with Russia to purchase the S-400 surface-to-air missile systems is likely to be concluded in an upcoming meeting between President Recep Tayyip Erdoğan and his Russian counterpart Vladimir Putin, scheduled to take place in Russian Black Sea coastal city Sochi on May 3.Commenting on the issue Foreign Minister Mevlüt Çavuşoğlu said Friday that Russia and Turkey reached an agreement regarding the purchase of the S-400 air defense systems, and negotiations on the price are underway."We agreed in principle with Russia on the S-400 purchases. The negotiations on the joint production and prices are underway," he said.Turkey and Russia have engaged in talks regarding Turkey's cooperation with Russia in the defense sector since the Russian-Turkish rapprochement in August 2016. However, the negotiations to purchase the S-400 missile system gained momentum in recent weeks.Daily Sabah on March 23 reported that Turkey continued its negotiations to buy batteries of S-400 systems.Meanwhile, as a NATO-member, Turkey's procurement of the Russian S-400 system has seemingly raised concerns regarding the compatibility and integration of these weapons into the NATO infrastructure.However, Defense Minister Fikri Işık recently clarified that Turkey will use the Russian systems without integration, noting, "Russia's S-400 missile systems will not be integrated into the NATO system." Işık said on March 15.On the other hand, Turkey's efforts to develop its indigenous long-range air missile defense system will continue, even after the procurement of Russian S-400s."We [Turkey] have already developed short-range ballistic missile 'Hisar' and are currently working on a medium-range missile project. Developments of our own long-range defense system are ongoing and we will be able to produce these systems within the next five to seven years," he said.The S-400 is a new generation of Russian missile system, which has so far been sold to China and India only.Last year, both China and India signed an intergovernmental agreement for the procurement of four regiments of Russian-made S-400s. Turkey will become the first NATO-member country to purchase the system.