OCSE monitors the war in Eastern Ukraine. They make daily reports about what is happening in the area.
On April 7 2017 OCSE reports:
Beyond withdrawal lines, but outside storage sites, in government-controlled area, the SMM saw ten towed howitzers (2A36) near government-controlled Khlibodarivka (65km south-west of Donetsk); and two stationary surface-to-air missile systems (9K37, 400mm) near Pryvillia (81km north of Donetsk).
9K37 is NATO code for what is known as BUK.
One of the reasons for Ukraine to position BUK surface to air systems near the ATO zone controlled by the Russia backed separatists is to destroy Russian supplied drones. These are used to get information about position and movements of Ukraine forces.